Not many I think it was around 12 or so but I did leave some just in case and received the same error:
Weird... the link seems to be working fine for others.
In the meantime, maybe try this link? I have it set to expire in 7 days, as a heads up.
If that doesn't work -- try going to the "Discover" section of Discord (bottom left, next to your account), and search up "Boss Rush Jam".