How can one add or reveal more control mappings?

I feel like I’m starting to better understand how to set up and take advantage of the control mappings at the bottom left of the MG3 window. That’s extremely cool. But I find I run out of available slots, even in the three-row view with its nine, and I don’t know how to add (or perhaps reveal) any more. Are there more slots available? If so, how would one get to them, and how many are supported in total?