A 1x1 rib is a multiple of two - 1 knit and 1 purl - so you need an even number. It's the easiest to remember.
A 2x2 rib is a multiple of four - 2 knits and 2 purls - so you need a multiple of four (80, 84, 88, 92). If, for example, you co 86 then you would start and end with knits so the pattern wouldn't work out.
If you choose to do an off rib number like k3p2 you'd have a multiple of 5. Whatever the total number is in the pattern is your multiple. CO accordingly.
If you are knitting in the round you just CO according to the rib you are doing and knit away. If you are knitting flat then you have to learn what the stitches look like because you can CO that off number I mentioned above.
It took me awhile to figure this out so I hope my learning experience helps you.