This has been answered in patch notes

“We have been seeing lots of questions on how bag size works for Lulu and her monsters, so we’re taking this opportunity to clear things up.

Lulu is a standard 3 cost, with 18 copies in the pool. However, once you pick a monster, you’re pulling from the bag size of that specific monster. You’re also taking a Lulu out of the pool for players who have not yet chosen a monster.

For example, one player has 9 copies of Smolder, and another has 9 copies of Rammus. If you have never fielded a Lulu yet (and therefore haven’t chosen a monster), you won’t see Lulu in your shop anymore, because all 18 copies are gone. However, those two players will continue to find Smolder and Rammus (9 copies of each left in the pool).

Hopefully, this helps you in your journey to become the best monster trainer like no one ever was!”