How many b1 fire extinguishers must motor boats 26 to 40 feet have aboard?

Motorboats between 26 and 40 feet must have two B-I extinguishers.

B-I is either 2 pounds dry chem, 4 pounds CO2 or 6 quarts foam.

According to US Coast Guard regulations.

NOTE: When a fixed fire extinguisher system is installed in the machinery space(s), it will replace one B-I type portable extinguisher.

Different regulations may apply to uninspected passenger vessels, including those "carrying not more than six passengers, including at least one passenger-for-hire" and for vessels in "international waters".