Steppers:
Regarding the software architecture there is no hard limit for the count of steppers. But all on creating stepper pulses is done in one and only one interrupt. So the more steppers are active the longer this ISR will run. And computation time per stepper increases if the stepper is ramping down or up. So at the end it is a question of processer load. This is especially a problem with slow processors like the standard AVR series. If things get bad the runtime is longer than the time to the next IRQ.
If you use softLeds ( bulb simulaton ) things get worse, because creating the PWM is done in the same ISR too.
That's why 'out of the box' the number of steppers is limited to 6 by a #define in MobaTools.h
#define MAX_STEPPER 6
This works fine even on an AVR.
If you don't use softLeds and don't let run alle steppers at the same time ( especially with ramping up/down) you can increase that #define to be able to create more stepper objects.
Buttons:
The MoToButtons class can manage up to 32 buttons within one instance. Default is max 16 buttons. If you want to use more than 16, you must insert a line
#define MAX32BUTTONS
before including MobaTools.h
That should be independent of MobaTools, but you need a processor with 9 analog inputs ( or use an external ADC.