Assassin´s Creed: Brotherhood Message Board  

I have 20 so far, but I was wondering exactly how many are in the game. I've checked all the Art Shops and I bought all of them.

Well, then take a look at all the painting frames in your hideout. Is each frame "filled" with it's associated painting, then you have ALL of them and don't have to worry no more If there's still a spare frame, then you have to visit the artists again.
I think you also get portraits of certain persons you assassinated, but I didn't check those.

If there are still spare frames in your hideout, despite having bought every painting from the artists, then you'll get the missing portraits after killing your target, which means just continue the storyline.