Have you tried reducing the number of units per formation? It could be related to the number of bots being spawned in, rather than the number of players perse (especially if it's directly after warmup).
I also assumed this could be the case, but even with significantly less bots per formation, it still crashes at 9. And when you vastly increase the number of bots per formation with <9 players, it still does not crash. I believe there is just an error in the code
There were few fixes made addressing similar issues. Are you still having the same problem on the latest version of the game? If so, could you also give us the Bot multipliers, scene name, player amount etc. data as well?