Hello.
As Jim mentioned, all of your servos will receive the same power bus voltage. 12V is almost certainly too much for SG90 servos and would likely damage them.
Jim’s suggestions of an RC battery pack is a good starting point. As he alluded to, if you can’t find an adequate supply to power all of your servos that would work for your project, you can separate the Maestro’s servo power rail into multiple banks and power them from separate power supplies by carefully cutting the trace on the underside of the board. You can find more information in this post by Jon. You might consider splitting the Maestro power rail into multiple banks (or powering the servos separately from the Maestro) even if you power them all from the same supply since the traces are rated for 6A.
Brandon