> is sharing a house illegal, or is it only illegal to separate leases for each room?

Too many unrelated people living in a housing unit is illegal. Here’s San Francisco’s version of this law which was used to shut down house sharing companies such as HubHaus; see definition of “family” https://codelibrary.amlegal.com/codes/san_francisco/latest/s...

The article also mentioned dormitory-like “group housing” apartments (which differ from housing units in that they don’t have a separate kitchen for each unit). San Francisco is pretty enlightened in that it allows group housing in many zoning districts, but even they have group housing density limits and now common space requirements which are designed to prevent much group housing (see definition of “group housing” https://codelibrary.amlegal.com/codes/san_francisco/latest/s...).

Meanwhile, people build tiny homes to "solve homelessness"...

https://www.youtube.com/shorts/yJsWjulAonc

Typically it’s two per bedroom plus one. The max occupancy is set on the lease.