alright so I'm not affiliated with RGL whatsoever and I don't really know what's going on in their heads but I'm pretty sure the "stv overpopulation" is a big reason why this rule exists, for those who are wondering.
A couple seasons ago me and like virgil and um wanted to cast a match of some of our friends, and we found out later that apparently it was also gonna get streamed on TFTV (or something like that I don't really remember). It turned out that we filled up the stv, so the casters on the big stream couldn't get in, which delayed the cast. After they managed to contact us, we hopped out only for the STV to get sniped by some randos (idk who), so the other casters still couldn't get in and now didn't know who to contact to make space. As a result, I'm pretty sure the stream didn't actually start until the 2nd half of the match, after the teams ended up switching servers.
I'm pretty sure this was the main point of the whole casting coalition--so different casters didn't accidentally clog the stv for other streams, and if they did, they could be easily contacted. And I think some of the logic there is pretty valid.
Now like a bunch of other shitty grey-area RGL rules, they just leave room for punishments to be misapplied. That is, if every known stream/caster is in the stv already and there is no risk of people getting blocked out, then there really shouldn't be any sort of punishment administered beyond a slap on the wrist and a "let us know in advance next time in case some shit happens." This rule also helps push the whole RGL exclusivity casting bit, which I hope everyone at RGL knows is exactly as stupid as it sounds, and mistakes like this end up pushing that narrative.
tldr; stvs can have pretty small player capacities if u didn't know