When the mystical plushies were still a thing, I can't remember if the value was $20 or $40 for the 1 and 2 pet slots respectively. i want to say the rate has been 1k CSC = $10 forever now, so it might have been $40/$80 then instead cause they were 4k/8k respectively.
in any case, the way slots are done now is different. You either buy a slot for 1.5k CSC that's currently at a cap of 55, then you either buy from the millionaire center til you hit that cap of 25, then you've got loyalty totems you can redeem starting after 2 years of being on the site (which afaik has no cap), or you get them as rewards for events and such. Basically if you've been on Subeta for a long ass time, it's easy to have racked up pet slots, but otherwise starting out it's still 5 for normal accounts, 10 for GAs. and I want to say if you unlock 10 from a GA ever, you keep those extra 5 pets forever, you don't have to get rid of them after the GA expires.
so it goes 5/10 starting out, then you can get up to 60/65 from CSC purchases alone, then 85/90 from buying from the MC, and from there, over time you can wind up with 100+ pets depending on getting prizes for events, loyalty totems, and if you're an old timer who redeemed those plushies and early loyalty totems, it's likely you have the ability to get 100.