In some cases you may want to constrain the total number of units in play, perhaps because of performance reasons, or because some type tends to proliferate more than is desirable, or because your game concept requires a hard limit on the number of units. You have several ways to do this.
Xconq does give you several parameters that put a simple cap on total numbers, either by unit type or for all units, and per side or for all sides together. You can also define a material type that is essential to the creation, completion, or operation of units, and make that material be hard to come by. Iron to make ships, gold to pay armies, or food to feed armies could all work this way. If the only source of the limiting material is an initial supply in a starting unit, then this is a hard limit; if production of the limiting material is slow, then the limit is softer but still very real.
Limits on unit quantities have some interesting uses beyond the obvious ones. For instance, a useful type that is limited to at most a single instance could be a sort of "football" where the side that has the one unit finds itself being chased after by all the other sides trying to get it. You could make a WWII-era game with "Oppenheimer" as the only scientist who knows how to make an atomic bomb (I know, it's not realistic), and have the different sides trying to kidnap him.