How many fixtures can you vent on a 2 pipe?

Under the uniform sanitary code, a fan 2 can handle 24 connecting units (USA). In a residence, a bathroom of 1.0 U.S., shower of 2.0 U.S. and a bathroom of 1.6 gallons or less 3.0 U.S., over 1.6 gallons 4.0 U.S. bathroom.