There is a possibility of a limit being set in the near future. However anyone who is a member of GSRA are allowed to request customs. And not too many of the members are active. with each rabbit made, is another representation of GSRA and sort of a "Hey! This is unique! Do you want one? Then join.

" type of thing.
As to the available slots, I may take those done due to the fact that I made that when it was just me and teresa8oats as artists. We were both busy and it was a lot to take on, but now that there are more artists and they are more active, they may take on customs. As long as they are able to get done in a fairly timely way and that they are not hogging from other artists that want a chance.
But yes there will be a bit of a limit, in future, although now everyone is just getting their bucks and does that they want to breed for when the Nursery opens (Which is VERY soon.)
EDIT: I have just added this to the custom post.
1-4 custom rabbits per user ( When open)
Artists get to decide how many customs they take on.
Status/sayings: Life being consumed by rabbits and dancing. Hi I am Baby bunnies! but you can call me BB
I love to do art. Before December 2014 all art that I did was with a mouse. I now work with a Bamboo tablet. Also please note that I expect something in return for customs, I put time into every piece.
I own rabbits! I love to show my rabbits and and have gotten many first place ribbons, a lot of Best of Breed, Best Opposite Breed, Best of Variety, & Best Opposite Variety awards. My rabbits have also gotten Reserve in Show and Best 6 class. I, personally, have gotten Reserve Champion and Grand Champion awards. I take part in Rabbit Royalty! If you have any questions about Rabbits then please ask and I shall answer as best that I can.
Info on how to properly raise rabbits and show them. Rabbit Blog
Rabbit Q&A ~ Baby bunnies! and teresa8oats
