afaik there is only PLOT_UNIT_LIMIT which governs all unit types, which allows by default one unit per tile, or one unit with attack and one unit with no attack
there's stuff in the lua that ejects units you could look and see if those can be blocked... but it's going to be messy in game. ever had a coastal city with a naval unit, a garrison, a worker, and then a new unit just got built and a great person was born there also? not so simple to select a unit in that stack off the map