The 312 cannot support any expansion racks so it is limited to 1 rack.
The 313 and 314 support up to 3 expansion racks for a total of 4 racks.
Each rack can have up to 8 modules. But with the 31xC, you cannot use a module in slot 8 of rack 3 (the fourth rack). I believe there is a current limit on each rack that may limit you to fewer modules depending on the modules you use.
This information is in the catalog. Click on the "Design" icon and scroll down to the "Extension" section.