In the mid game, you will be able to run 6 Duelists assuming you find the Duelists you need.
Additional comp tips
Jax is the main carry of the comp, so try to stack him with three items. His items are very flexible, so you don't have to worry too much about getting perfect items. Defensive items in general are good enough on Jax as he has Duelist and Divine to give him more damage.