Yes 15-17 tropheus would be ok for a 55gal. Just make sure you have some hiding places in there for the ones that get picked on. They will definitely breed in a tank that size, I've got that many in a 60gal and they breed on a regular basis. I personally like Ilangi, ikola kaiser, green murago, moliro red, and nkonde yellow. If you are just starting off then duboisi are the easiest to care for.
If I going to get a group of them, but I am able to get like 4 at a time but I am getting it from the same breeder. It's literally same batch will be same size but I am just short of cash. Can I do that? or does it have to be a complete group at the time?
Save your money, until you can afford to buy them as a group. Buying four at a time is a recipe for aggression issues, likely leading to deaths. You could also save money by purchasing juveniles, which are typically cheaper.
As most people have already said, multiple adds of small quantities of tropheus is a disaster. I know it is overboard, but I am an advocate of getting the whole colony setup once and leave it alone size wise. Just like food, don't change anything.
I think you are taking a big risk with such a small tank.
I would advise on a larger tank for these guys and agree with everyone that you want to purchase your colony and introduce them as a whole. This can still come from various sources but remember that tropheus vary geographically and each population tends to be unique and forms a type.
I would avoid the possibility of hybridizing across types so try to avoid that situation.