As of right now there are a few limits based on plot sizes other than space and total item limits.

Gardening plots: 1 for small, 2 for medium, 3 for large.

NPC's placed inside of the house: 2 for small, 3 for medium, 4 for large. There are a total of 3 different NPC's that can be placed right now but with the 4x limit in large that might change in the future?