Whatever float is available on the market can be made available for short sellers to borrow. That can even happen multiple times: ie short interest can exceed

Whatever float is available on the market can be made available for short sellers to borrow. That can even happen multiple times: ie short interest can exceed 100% of the float. Or even 100% of the market capitalisation. With stock futures, you don't even need to borrow the stock to (effectively) short it: anyone with enough collateral can write stock futures, whether they own the underlying stock or not.