Necromancer

Without any points in Drone Count, a single Necromancer can have a maximum of 22 drones.

For every point spent in Drone Count, you can have 2 more drones. To easily calculate the maximum of Necromancer drones with r points in Drone Count, you can solve 22+2r. With 7 points in Drone Count, you can have a maximum of 36 drones.

Overseer, Overlord, Manager

You can have 8 drones as these 3 tanks before reaching the limit.

Unlike Necromancer's drone limit where there is a Drone Count stat replacing Reload, as shown in the above section, you cannot upgrade Drone Count for more drones.

Overtrapper, Hybrid

You can have 2 drones as either of these tanks before reaching the limit.

You cannot change this limit.

Technically Hybrid isn't a drone tank, but I'm putting it here for completeness.

Factory

You can have 6 drones as a Factory before reaching the limit.

You cannot change this limit.