What is the occupancy limit for a 2BR townhouse in South San Francisco?

I live in a 2BR townhouse in South San Francisco with my wife, 3 children (8,6 and 2yr old) and my mother who helps us babysit - 6 total. The landlord pretty much threatened that we will get evicted because there's a limit of 5 people that can live in a 2BR apt. It concerns me that she told me this after I had asked to get our clogged toilet fixed (the plumber later found coins and small toys were in the toilet so the landlord asked us to pay for the new toilet, which I agreed on, knowing that my 2yr might have done it). Our landlord knows about our living situation for 2 years now since the birth of my 3rd child. Why now? I have heard that up to 6 people can live in a 2BR apt but the landlord insists that the limit is 5. We've been living here for 9 years... Pls help!