Assault Vehicle Factories are first available at HQ1 and have a total of 3 levels that unlock Fortress, Hammer and Zeus tanks. Unlike Barracks, the Health and Armor of this building is far superior to administrative buildings like the Supply Center. This makes them relatively difficult to snipe with avia, which means they can be placed in areas of the base that are less protected by Anti-Air.

There is not a particular recommended number of Assault factories, but for rushes the optimal number is when the resources that you need for production are roughly the same as your Supply Centers production, allowing the most time-efficient resource to unit conversion when time matters the most.

It is also a valid strategy to produce Hammers or Zeus from one factory only when one plans to go for a high-tech approach with lategame units as this doesn't slow down base development too much, while also leaving enough resources to tech up in a reasonable time.

Furthermore, Assault Vehicle Factories require at least 1 Barracks in your base before they can be built and in turn unlock the remaining production buildings. This means that it is always a good idea to build the Assault Vehicle Factory when teching up to HQ2 as it allows a faster transition into the targeted unit type.