Plans aren't set in stone yet, so I can't give a definitive answer to how many total characters there will be. Rn there is 14 + the 3 coming next build. Then there's atleast 6 more waiting to be introduced (2 for Stephen's route, 1 for Raphael, 1 for 'Vincent's', 1 for Chase's, 1 general.)
As for romanceables, well each route will probably be different - and again it's not set in stone yet, things can and probably will change; but the current plan is:
Raphael's route is just Raphael himself.
Chase's is probably just Chase with the possibility of someone else in the future.
Stephen's route: from Eric's pov will be nothing fancy but you'll be able to do things with two yet to be introduced characters with one of them previewed in the recent devlog. From Stephen's: it'll be between him and Robin.
'Vincent's' route hasn't had the 2 "dateables' formally introducd yet, so I don't really want to spill - but it'll be blatantly clear who they are when 'vincent's' ch2 p1 drops.
There also won't be any romance content with Vincent.