Considering that the average attendances for SuperLiga (fist tier soccer comp and probably the most popular sports comp over there) are normally well under 10k (with the exception of Red Star that normally hovers around the 10k mark, with an average of 12489 for the 17/18 season and an average of 10508 for the 16/17 season) I highly doubt that they'll get very many people through the gate.
Who knows maybe if the WP work with Red Star Belgrade (and any of the other sports clubs in Belgrade that are open to working with them as well), get a ton of publicity, and market it really well they'll get a surprisingly good crowd, but realistically I wouldn't be expecting more than 2-5k and if things don't go their way (or they balls it up) it could be even less than that.
Here's the link to SuperLiga averages if anybody wants to have a look.