As of Patch 2.6 (released August 28th 2014), AI Station 404 has been replaced by AI Station 205 in standard online matches. However, both maps are available in bot or custom matches, and both feature Droid Releasers, though the mechanics are slightly different between the two.
In both maps, a maximum of 3 Humming Droids can be spawned at a time from one team's Droid Releaser. Droids will be restocked one at a time after a certain interval, as long as the combined number of active and stored droids don't exceed 3. On AI Station 404, it's 10 seconds for each restock, while on AI Station 205, it's 15 seconds. The restocking timer doesn't start until there's a "slot" open, i.e. if you have 3 droids active, the timer doesn't start until at least one has been destroyed.