This might have to do with your Steam level, but I don't remember all those conditions anymore.

maybe u need to buy one that lets u put more shoe cases on ur profile than u can do rn

I can't remember but every 10th level you get one showcase to display up to either 13 or 14 (Level 130/140). I could be a little bit wrong but this is at least the gist of things. There is information somewhere on Steam about this though in more detail.

Thank you guys for the replies, I think I have it figured out. I bought 3 showcases and my level is 14 so I'm getting 4 slot's right now. I ended up using different showcases than the ones I bought but the number of slots is at least adding up. I think I need to level up my account (next year the current badges don't fit my profiles theme) if I want to display the other showcases I bought or buy another one to get 5 slots.