In MainStage Layout Mode you'll want to make sure to select the number of keys that your keyboard has, as well as it's bottom note. You can type it in, or you can choose "Learn" and press your bottom key.
MainStage classifies middle C by default to the Yamaha standard or Roland standard. You can adjust this in MainStage 3 > Preferences > MIDI...
This does not impact the sound or playability at all- however you can change it if you need to.
If you are in Edit Mode and you click on the layers or your channel strips, you'll pull up the bottom "Layer Editor" section in MainStage. Here you will always see the maximum notes possible in MIDI, just in the event you wanted to transpose your keyboard down or up multiple octaves- or in case you had an extended range keyboard. However, the keyboard in your workspace is what matters.
If you go into Edit Mode and you do not see layers floating above the keyboard that match with your channel strips, you'll want to check this out in Layout Mode. There is a box you'll want to check that says "Display Keyboard Layers" and allows you to set how many you'd like to see.