Hi Nour,
Since you are not too sure what you want, I would suggest you start with a small system (I mean half full system/case) and get some experience. With that experience you can then decide which other modules you would like to have and then can start to fill up the space.
Leave more space free than 10 HP (that's just about one or two modules). You definitely need more space reserved for future use otherwise you end up soon with buying another case :-)
Kind regards, Garfield.
For review reports of Eurorack modules, please refer to https://garfieldmodular.net/ for PDF formatted downloads