In Star Trek Beyond, Krall's back story states that only 3 were left. So who are the majority of his army? They didn't seem to be drones or machines. This was unexplained as far as I could tell.
Krall states that they were machines (drones) in his fight with Kirk in the air duct at the end. This helps explain why they blew up when the Beastie Boys were pumped into their system.
He says something like this (remembered quote):
Only 3 of us survived, on this alien planet with advanced technology, drones, and life enhancing tech.
They are the drone-like workforce native to that planet, who were abandoned by whatever smarter beings used to direct them. Krall took control of them.
The "3 of us left" comment is about Krall and his crew, not the species (as Krall was actually human). Krall and the crewmates who survived with him used more mostly-unexplained alien tech and, either intentionally or as a side-effect, came to look like that species.