The transition point is not the number of properties just like it isn't the dollar amount of income. The transition point is whether or not, in the method they are using, they need to contribute their own labor beyond perfunctory business decisions and things of that nature. If someone has one unit, but due to inflated rental prices it can be fully maintained and mostly managed by employees, then the owner is bourgeois. Realistically, you need more units than that, but it's not about a pre-defined number of units (which by the way isn't accounting for variance among units). It varies place to place based on rent inflation, labor laws, housing regulation, taxation, etc.