That's the number of people who received problems on that level. I would guess most of them are not working on these problems (especially Level 0). For example

That's the number of people who received problems on that level. I would guess most of them are not working on these problems (especially Level 0). For example I got Level 0 set of problems, enjoyed the reading, but decided not to work on these problems, because I'm just not enthusiastic enough about diving deep into cryptography. I guess most of 4362 "players" on Level 0 are in a similar position.