In using resistance bands IN COMBINATION WITH exercises - such as dumbbells or barbells, or weight stack machines --and trying to decide which strength bands... a basic guide to use is if you can't bench press over 300lbs, then you don't need anything over 50lbs per band per side - to mean on each side of barbell; .... working backwards from that... if you bench is around 220 or so, then I'd suggest about 20lbs per side...again only a suggestion - if the bench is less than 200 - I'd suggest using 10 - 15 per band per side/hand... I use bands IN COMBINATION with my workouts... I attach my own band, my product, to all exercises...barbells, dumbbells, and all weight stacks.
If you are going to use bands alone, without attaching them to equipment... you are going to have to do trial and error... buy a resistance - say 20lbs, and go from there.