Good evening
What does the little box next to a player’s jersey stock mean? Bear in mind that I have space in my warehouse and in my total stock

Hello, you can only sell the shirts of 7 different players. I think you have shirts for more players than that, so the players with the fewest shirts to their name aren’t for sale, and there’s a little box for them.

Personally, I can only sell kits for 5 different players – does this number increase depending on the workshop’s level? (Mine’s at level 4)

No, at level 4, you can sell flock-printed shirts featuring 7 different players. However, you need to be able to produce them. If you want to produce them all at the same time, that means you need 7 flock-printing workshops.