The command was removed because at the time t3 donators had access to all mob spawners, and yes it got abused by monkeys thinking it was cool to make irongolem spawners and farm them for iron to sell.
So it was stripped away for everyone in survival map, however I do plan to put it back albeit without access to enderdragons,withers, irongolems, etc
The old map doesn't have these spawner restriction currently, however the warning is just the same, anyone doing stupid stuff with mob spawners in old map will lose the perm.
Also its 30k for spawners, all donators have access to the kit money, so buying spawners should not be an issue.
As for the limit on number of bought spawners a player can have, its probably around 4-8, though if said player is in a town and that town has a spawners, then it goes by the number of spawners in the town/area which I'm going to say should be at the most should only be 16.