You can have many more than 12. Each MFD displays 12 modes per page (14 for the Space Shuttle.) You can select the next page (next set of 12 MFD modes) by pressing SEL a second, third, fourth time, etc. ad nauseam. You can select which MFD modes you want active and selectable by activating/deactivating them in the Modules tab on the Orbiter launchpad (check/uncheck the boxes.) With some checkbox wizardry, you can even arrange them in the order you want.
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