Puerto Rico and DC don't get to vote in Congressional elections. Well, not for seats that get votes, anyway. But yes, if PR and DC were granted statehood, we'd have 3 or 4 more Democrats in the Senate, and some number of new House districts, the majority of which likely sending Democrats to the House.
That's why Canada would be the 51st state. Only 2 senators for all of Canada, so not much increase in Democratic representation. Representatives would of course lean Democratic, but if Trump engineers a collapse in Canada's economy, some voters may be willing to vote Republican if they're promised a bailout, given the fait accompli of annexation. Plus, if the NDP sticks around, that's fewer Democratic representatives. Sure NDP would probably vote with Democrats in most situations, but it might be easier to peel their representatives away for crucial votes.
Greenland could probably be induced to vote Republican fairly easily, and so send 2 Republican senators to Congress.
I've just spitballing, of course - in the current situation the GOP is not in the position to throw away any advantage in the House.
