The usual way is one invitation in the middle of each sheet, then scissors. Most of the paper is wasted and no two cards come out the same size. There's a better way that takes a minute.
Turned sideways, two 5 × 7 cards fit on 8.5 × 11 with room to spare. Twelve invitations is six sheets instead of twelve. The trick is getting them lined up, at exact size, with something to cut against.
Cut marks are short lines in the margin showing exactly where each cut goes. Line a ruler or a paper trimmer up with them and every card comes out identical. A small gap between the cards means a slightly wandering cut takes white paper, not the next invitation.
Print at 100% ("actual size"), not "fit to page", or the cards shrink. Card stock around 80–110 lb feels like a real invitation and goes through most home printers.
Free, no sign-up. Works for business cards too: ten to a page.
Make a sheet of invitations