before 9.4, sprites use an indexing system using a short variable, which can only go as far as 65.535, its something that is impossible to work around, if you want more then that then you have to use 9.4+ client which uses an integer to index, meaning you can go up to 4.294.967.295 sprites
to have more then 65.535 on a version previous to 9.4 would require huge amounts of editing in the client, not even sure if possible