How many could emigrate/flee to the USA in the event of Finland being occupied?
Let us consider some ballpark figures. We might estimate 100 000 or so people fled West from Estonia during and after WWII to escape Soviet rule. As the Finnish population at the time was three times that of Estonia, we might say that under similar conditions of occupation that would make about 300 000 Finns emigrating during and after the war. We might up that number to, say, 400 000 if there is for example an official policy large-scale evacuations but I'd say half a million would be pushing it.
Of these, a fair number would end up in the US given the history of Finns across the pond. But no way more than half of the total number, I'd say. Many would stay in Sweden and Norway or in the other parts of Western Europe, many would travel to other parts of the Americas. Canada, for example, would also likely see an increase of its Finnish population.
Counting Finnish Americans the way the US Census Bureau seems to do, I'd hazard a guess that in 2013 the top figure might be somewhere around one million, about 300 000 of those being post-1939 immigrants and their children and grand-children, etc.
I very much doubt it would be any more than that, and even that figure is probably much too high. Of the Estonian emigrants, for example, not nearly as big a part reached the US. Then again, Finns did already have a lot stronger tradition in America than Estonians did, so it evens it out a bit.