How many universes are there?

[wstein]

I had the understanding that space (the spacetime continuum) can't be infinite because it's expanding,

contrary to popular understanding, there is not one infinity. Infinities come in different 'sizes'. Also infinite is can expand/increase though perversely it doesn't get any 'larger'. Note that combining infinities can make a 'larger' infinity. Its better not to think of infinity as a precise number. Though not technically correct, it is easier to work with if you consider it an undefined really large amount.

and for it to expand there has to be some sort of 'quantity' aspect, that maybe what is infinite is 'where' it's expanding to?

It does have a 'quantity' aspect. That quantity in this case is 'infinity'.

I know it kills brain cells to contemplate something expanding into nothing. However in the case of universes, that is pretty much the case. Really, the size of the universe does not affect nor is affected by the nothing.

Infinity need not expand 'to' anywhere. One can just as easily get an infinity by dividing something. For instance, cutting a pie into ever smaller pieces. The total amount of pie remains the same.