For Canada, try this:
You get 12 states and 4 territories, or possibly more states.
Top row: Yukon, Nunatsik or Athabaska, North Hudson
Middle Row: Columbia, Saskatchewan (49° to 52° N, 114° W to 107° W at the southern portion), Manitoba, Winnipeg, West Hudson (green), East Hudson (orange), East Quebec (gray)
Bottom Row: former Indian territory, Ottawa (peninsula), Quebec, Nova Scotia, St John Island, Newfoundland
That may or may not work historically, depending on your timeline. This is based on my rough idea that the US settles on the 49th parallel with the UK after the War of 1812, and lets the UK have a sliver of access to the Great Lakes after the war, but the UK loses Oregon Country to 52° N, letting the US get another state, and also ensuring that Rupert's Land never becomes a draw for settlers. In the latter half of the 19th century, the UK sells Rupert's Land to the US, and the US continues the 52° N parallel to form additional territories, which eventually become states in the 20th century. Due to the climate, they are slow to become states and it drags on into the mid 20th century. Yukon becomes a state in the 1960s.
That reminds me more of what might happen in some grand redistricting of every state in the Union, rather than the aftereffects of an annexation. Your logic is sound; I just don't know of that ever being done before; it'd be hard to tell if this is possible without knowing the timeline.
If we're going for a pre-Canadian Confederation (pre 1867), you're likely to see all of the existing colonies gain statehood as such. So, PEI, Nova Scotia, New Brunswick, and Newfoundland would all be states; so would Upper and Lower Canada, along with Columbia and, depending on the PoD, a Manitoba of some sorts. There would be a large bias shown to those various preexisting colonies, which is a deal I think the Canadians would take. That would give them a large Senatorial representation to balance out the low population (the same deal that New England gets now, basically), which will allow them to have an outsized effect on politics.
Look at it as three regions: You'll have Columbia, Manitoba, and whatever other states are created in the Prairie Provinces and the Northwest Territories. Those are the territories which will develop a more American culture based on the low population and eventual immigration. You'll have the Maritimes: Newfounland, Nova Scotia, New Brunswick, and Prince Edward Island. They will maintain a small bloc and will likely be the least Americanized of the bunch. Then you have the center two provinces: Upper and Lower Canada (Ontario and Quebec). Ontario, while likely to be the strongest, will also have the greatest links to the US heartland (due to geography and nothing else). Quebec, owing to its character, will be the most dissimilar to the rest of the US, both American and former Canadian.
So, to start out, you have that wild card scenario. The 4 Maritime States with 2 Senators apiece, along with Upper Canada/Ontario, will create a large bloc. Quebec is a wild card in future politics, at least in the interim. This will eventually be balanced, in the west, with Columbia (I'm just calling it that; it can be named whatever), Manitoba, and a few other states eventually balancing the senatorial count while also, eventually, usurping all but Ontario/Quebec with population.
Starting from this 1862 map: (mostly for good basemap, and partially because I'm not sure what the PoD is):
I would find that this might be a reasonable assumption:
Red borders are ones that are easily changed; Blue borders should be the absolute minimum.
A. Your Sitka/Yukon State, with borders along the edge of the rockies and mostly concentrated along the coast.
B. Columbia, but only with its 54' 40" borders (minus former Alaskan Panhandle). Vancouver Island is part of the state; there's no reason to separate. The two colonies were separate at one point, so there is potential here.
C. Prairie Province State. Could end up being split in half or, I find more likely, have some of its territory given to Manitoba.
D. Manitoba, without access to the sea. Doesn't seem likely, as the Nelson river zigzags around its border and Churchill would be nearby. I think Manitoba would expand at the expense of C and H, frankly.
E. Upper Canada/Canada West/Ontario. I don't know of any separatist sentiment, and part of the impetus for the Canadians to join the US would be remaining large states with lots of room to grow. If they are worried about the Senate, keeping the maritimes separate solves that.
F. Lower Canada/Canada East/Quebec. Same as above; Don't know of any reason to split them up, other than power struggle between Montreal and Quebec City. Seems more likely that they'd remain united so as to prevent the erosion of their culture (rather than being split up and having half remain French and the other half slowly Anglicizing).
G. Maritimes. Nothing of note, but all 4 would remain separate as a concession to keep them in.
H. North* Territories. This is the absolutely smallest I think they can be, although you might divide it in two if it incorporates more of the border area.
1. Could be a region that goes to Sitka or Columbia (A or B)
2. This would work as a straight lineborder, or some could go to A and some to H (and some to C if the border moves north). I'd argue that, as the majority of the population would be in the former Alaskan Panhandle, with some in Whitehorse, that the majority of the territory in the north should go to H. It'd be easier to make H a state, the larger it is.
3. A small section that would give C a more natural border; it's mostly falling hills and mountains, making a straight line over depressions to get to the next ridge.
4. A lot of land that could go to H or C/D. Due to natural resources, it would likely be later appended to the latter two.
5. The aforementioned region that would give D control over the mouth of the Nelson River and Churchill.
6. Potential Borders allowing E an outlet to the Hudson. Having some minimal amount of territory seems likely, rather than it continuously being locked up. One follows the 90 Degree West, while the other two just mark the line from the westernmost border of E, and one border heading from the northernmost point of E.
7. F would likely want most of this territory for their own, and would likely be part of the concession to bring the
Canadiens onto the Us side; they would likely be larger than every other former Canadian province, and would have a domineering position over the Hudson bay.
8. Labrador would be disputed between Newfoundland and Quebec (F). As Labrador's borders were ill definied, it is likely that Quebec would likely maintain at least a portion of Labrador, if not being granted the whole thing. If Labrador does remain a separate (if reduced) polity, then it will likely be appended to Newfoundland as per OTL; not enough population to remain independent.
Anywho: that gives you 10 states by 1900, with the potential for 2 more in there. One other border I didn't cover (as it was getting cluttered) would be a potential border between C and another Prairie Province state along the Peace River (or the former Rupert's Land boundary, but watershed borders are... difficult to delineate). Edmonton would be along that, so you could see a bit of a Kansas City situation developing along the river. That would make C quite a bit smaller, and would require giving that new state the majority of the territory within 4 to maintain viability. That, and such a border might interfere with Manitoba's own borders. Still, there's quite a lot you can do.
