I might be wrong but i dont think you have to apply for any sort of licence or specify any sort of limit if you are using a propelant. If you are applying for a black powder permit i put "legal maximum" when i applied. That was a long time ago though and Health and Safety probably plays a bigger part now.
You can keep as many primers and balls as you like. I tend to make my own balls as and when i need to and buy primers 500 at a time.