In the Bartimaeus books magical entities are "killed" all the time. This is notably different from just returning to the "other place", they actually stop existing. As far as I know there is no mention of new entities being created in any way ever. So I would assume that their total number only ever decreases since wizards started summoning them. Bartimaeus also keeps encountering familiar faces, so I would assume that there aren't quintillions of entities to "use up" over time.
So the question is: How many are left? Is there a rough estimate in any canon material? Are they in danger of dying out? Or can new ones be "born"/"created" in any way?