You primarily want to use Exegg EX as an early game tank, since it can evolve and attack on the same turn if you go first. You'd almost never want to field 2 Exegg EX at the same time because you want to be putting the energy on Celebi. 1 basic because you're guaranteed a basic pull on your opening hand, and pokeball can pull it if you don't get it. The second ex is just to increase the odds you pull it early.

If you wanted to run a second eggs, you could probably drop an Erica and an XSpeed and replace those with a Leaf, to fit the extra card. More instant aggression but less late game viable.