Hi !
It doesn't matter how many items you have. You gain one half yellow star for each level of Entrance you have (no matter on what item, you can have 50 in one, it's the same as having 10 in 5). Since 10 Yellow Stars make up 1 EP, you need 20 Entrances to gain 1 additional EP (or 10 entrances in a show, since they are doubled there).
Any more questions ?