There's an excuse, in some cases, for owning more than one house—I know of at least one person who owns a home for herself and a second that's used by her disabled sister, for instance, and there's the classic case of someone who just inherited their parents' house but is too deep in the grieving process to have been able to empty it out and sell it off yet—so I think we do need some flexibility. 2-3 houses seems like a logical upper limit. We do need some house-sized rental properties, and better to have them separated between multiple small landlords than owned by corporations who see the additional tax as just a cost of doing business.