Within Saranity you will have the option to build 2 harvester units. The amount of harvesters you can build is determined by all the buildings you already constructed. Some Buildings will allow additional units to be constructed while other buildings allow new harvester classes.
Building 1: Barbarian Hall
Spawn cost: 1000
Unlocks:
Building 2: Magic Well
Spawn cost: 2000
Unlocks:
Building 3: Ultimate Shrine
Spawn cost: 4000
Unlocks: