Optimal number of Single Family rentals?

[Paul C.]

Anyone have an opinion on what the optimal number of single famly rentals to have is?

Let's say the goal is to achieve unlevered returns on capital of 7%, but be relatively hands off via a good property manager.  The ideal extent of involvement would be a few hours of bookkeeping each month and fielding the occasional question from a property manager on how something should be handled.

Clearly, 1 or 2 homes would be too few and not allow you to smooth out cash flow over time.  But is there a point where it becomes too many?

If you had unlimited capital, how many would you ideally want to have before focusing your investment efforts elsewhere?