I hear that too many global variables can bog a game down...

Is that true? If so, how many is too many?

Having a bunch of variables shouldn't be a big deal.

Doing something to a bunch of variables each tick is.

No, global variables can probably be entirely ignored from a performance point of view, they are so fast as to effectively have zero impact. A pretty much complete list of things you should worry about for performance is here: https://www.scirra.com/manual/134/performance-tips - anything not listed there is probably not a concern.

But still I recommend to use local variables, since too many global variables will be very confusing

Ashley and everyone - thanks.