how much can you put into a 37c envelope? Like how many ounces...or at least how many toploaders.

Just at a guess .... 1 toploader + mylar + card is probably actually over 37c in weight but won't get pulled for insufficient postage very often.

You can try the
government postal website

postcalc . usps . gov

we aren't supposed to put links to other sites so you'll have to type that in less the spaces :)