1000-10,000 is the short answer. For a mech, upper end or higher is good.
Dont listen to Karotte, the Glaive and all other cloakbots are 1000+ polygons (more for the warrior and zeus) and they are spammed by the dozen. Polycount is not a huge bottleneck for modern PC's, pathfinding will always screw over your PC before polycount does.
If the glaive costs 65, and has 1k polygons, then the mechs, which cost ~100 as much (~5000), can afford 100,000 polygons. I am not joking, you will easily see 100 glaives at a time in some games. That is not a reason for lazy modeling or using cinematic levels of detail (These are still game models, so no 1000 sided cylinders) but practically, dont worry too much about polycount. If you're making a Mech unit though, it should look _fantastic_. Though as long as its better than the current ones, obviously, thats a start.
Glad to have more contributors!