This unit option limits the number of units a player can have of the unit overall. It just prohibits building beyond the point, not capturing or starting with more. See Piece Limits for the number of pieces that came with each version of the game.
maxBuiltPerPlayer is the number of this unit that can be BUILT by a player. If they have fewer than this number they can build more, if they have more than this number they can not build more [this does not cause excess ones to disappear]
<option name="maxBuiltPerPlayer" value="3"/>