Thanks! :D
They spawn every 5 seconds. The amount that spawns depends on the amount of factories remaining (1 sabot per still standing factory) (so on the first level you get 1 sabot every 5 seconds), so to avoid getting overwhelmed you are supposed to sabotage them as quickly as possible.
Yeah, sorry about the controls, I didn't have a numpad to test it on. Maybe if you toggle num lock it works? The hjklyubn keys I took from Cataclysm DDA (vi keys + some additional ones for diagonal movement). The qweadz[x/s]c keys are supposed to be more intuitive on a keyboard as that set of keys more closely resembles the shape of a numpad. I'm thinking about adding the option to navigate by the relative position of the mouse to the player, which is what Gnome Robots does. I also should add the option for players to set their own keys bindings (which is something still have to implement in any game I've ever made XD).