Looks like at least 750-ish boxes in stock:

"Please note: The maximum quantity allowed for the FUFP100CG45 -- FP-100C Professional Instant Color Film Glossy, ISO 100, 10 Exposures, 4 x 5" is “747”. Please select a smaller quantity. "

this is the message when trying to buy 999 boxes of it... ;-)

Adorama doesn't seem to have real-time stock quantities... I had 10000 boxes in my cart @ $370k and didn't get any warnings regarding stock levels.