USCG Requirements - Fire Extinguishers
I can't help with the regulations for a boat your size, but would recommend reaching out to your local Coast Guard Auxiliary and arrange a Vessel Examination.
This is a great courtesy service where a qualified vessel inspector comes to your boat and checks all the Coast Guard vessel requirements -- including fire extinguishers. There's no enforcement of penalties involved and you receive a Vessel Safety Check (VSC) sticker. Having one of these goes a long ways if you are boarded for a Coast Guard inspection.
Best of luck!