Limiting the number of units one can produce of a specific type is a bad choice. Units should have particular roles that make them able to be defeated by specific tactics or unit compositions. For instance, horse archers could be good at hit-and-run, but when fighting the more cost-effective foot missile soldiers, though they may win in some brief engagements, the cost would strain an opponents eco. Also, foot archers could have good range and decent damage, yet due to line of sight restrictions, they could not fight to their full potential without other units doing recon.