Usually you would go for 10-20 mmHg if you don't already have varicose veins or other problems - if you're just using them for prevention. If you have had things like venous stasis ulcers or varicose veins or that kind of thing, you would go to 20-30 mmHg and probably I would recommend meeting with a fitter to make sure you get the right fit/kind. Around where I am, that level of compression and higher will require a doctor's order. Often extended health benefits will cover them.
Otherwise wait until they go on sale and just get the OTC lower-compression ones from any old pharmacy. They're often sold as socks to wear on planes, or "sport socks."