How many houses are there in an estate?

It depends on the estate.

All estates have at least one - the house for the estate's owner.

If that house was relatively small (relatively meaning 5000-7500 square feet - remember, we're talking about very rich people here) there would be a number of additional houses for guests and servants. Depending on how rich the owner was, there could be twenty or thirty of these smaller houses.

The other end of the spectrum is George Vanderbilt's Biltmore Estate, in Asheville, North Carolina. Biltmore only ever had one house, but the house has 250 rooms and the servants' quarters are on the top floor.