Answer: Three (or more correctly four).
Three people need to be foreigners. More specifically there needs to be one Australian, one German and one French person. There also needs to be one American bus driver to scream instructions at the foreigners.
San Francisco buses have bike racks available on most of their buses. These racks accommodate two bikes only, so if the bus you want to jump on with your bike already has a full rack you will have to wait for the next bus or cycle up that dreaded hill you want to bus over instead.
If there are no bikes in the rack you will need to unlatch the rack from the bus. There is a silver handle in the middle of the rack that you squeeze towards the upper part of the rack to disengage the locking mechanism. You lower the rack, place your bike on the rack and secure it by pull the metal pole (left side of rack if facing the front of the bus) up over the wheel as high as possible.
See information on the MUNI website for a photos, video and instructions.