Is the usb connector only for programming the mega or can I use that to send/receive midi info via computer. Or will I need another device to send midi over usb?
Sorry, I'm no Midi expert,
but I managed to find something,
Have you tried searching for "midi over usb arduino"?
Also, how many switch/buttons can I use with this device? 54?
With 54 digital I/O pins
There are many ways to monitor keys / buttons, if you use a button for each entry will be the least possible. But if you use an array configuration the amount will be much larger.
I know you're getting started, but please try to put your questions on Google, this already helped me a lot in the beginning.
And, please try to ask one topic at a time, when the questions are very basic or of varied topics, few people will be interested in answering.
Good luck!