Ya. Its high tho. If you are hitting that limit, thats not good.
Actually, I don't believe there is a hard limit for uploads. Blockland instances aren't separated by account on the RTB server to my knowledge and thus I cannot think of a way to restrict the maximum size of the Add-Ons folder. There's a datablock limit for how many can get loaded into Blockland, but I don't think RTB has one. Just a logical limit, if you bypass it you may be told that you need to remove some.
Also, 900GB / 25 = 36GB per user, assuming 100GB is consumed by various other things on the server. You could probably get away with 40-50 before getting yelled at, since other people aren't going to upload that much data.