There will be a pop cap limit.
Different units will take a different number of population.
Mounts will be limited to how many units you have, for example your not going to be able create more horse than you have units that will ride them.
Siege weapons themselves for the most part (catapults, battering rams, ect.) have no pop limit (so far) as they require units to opperate them. Some siege weapons will be units on their own such as Tree of War and the Elven Ballista.