Trying to be as realistic (read: as close to OTL) as possible, which means limiting wars, annexing only not-completely-useless areas, and ensuring that states have reasonable populations, here's what I've come up with:
Texas split up after the Civil War: Texas, Lincoln, West Texas(?) = 2
Oklahoma & Sequoyah = 3
Cuba = 4
Santo Domingo = 5
Porto Rico (+ Virgin Islands) = 6
Nicaragua = 7
Philippines = 8
Marianas & Guam = 9
Pacific Islands (American Samoa, German Samoa, Micronesia, Nauru, Palau, Marshall Islands) = 10
Split up Texas ?! Them's fightin' words ...
No way would Texans accept having their state chopped up. Look at Virginia; they didn't officially accept the existence of West Virginia until 1971!