Fortnite mode selection?

When I choose a game mode on Fortnite
Solo match, group wedges and the like, I see small yellow keyholes with a number.

What does this information mean?

I use another account because I've never seen it before.

Can someone help me?

That means I mean you need at least a certain number of players which numbers are there

The numbers vary between
1-4. I have a screenshot on which, for example, in Duo + Team and creative mode, I see the lock and the 1 and I also had a lock with the 2 on the solo.

"If you assume that it is the missing number of players, I should be in a defined group, or am I writing and thinking nonsense?