There may be some confusion between "banks" and "sounds" per bank. From the MIDI spec:

Controller number zero (with 32 as the LSB) is defined as the bank select. The bank select function is used in some synthesizers in conjunction with the MIDI Program Change message to expand the number of different instrument sounds which may be specified (the Program Change message alone allows selection of one of 128 possible program numbers). The additional sounds are selected by preceding the Program Change message with a Control Change message which specifies a new value for Controller zero and Controller 32, allowing 16,384 banks of 128 sounds each.

Jeff, as I mentioned, I am not familiar with Mainstage, but in looking over their write-ups the following may steer you in the right direction:

https://support.apple.com/kb/PH13553?locale=en_US&viewlocale=en_US

Jeff

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