The numbers are N-P-K (Nitrogen- Phosphate- Potassium). Each number represents a percentage. So if you where to use 10-10-10 rather than 20-20-20 it would be exactly the the same if you used twice as much.
Nitrogen (the first number) is needed for leafy growth. Phosphate is needed more for flowering, and Potassium is needed all the time for root growth and some other stuff i can't remember at the moment. So during vegetative growth (before flowering starts) you want to give it lots of N and a bit of K (potassium). When it starts to flower you want to give it lots of P and still a bit of K.
Don't take this to any extreme; a vegetative plant needs P, and a flowering plant needs N. i.e. a 15-8-10 for veg, and 8-15-10 for flower would be good (i'm just pulling these numbers out of my "donkey", they are likely not available at stores)
But remember, these are just ratios. 15-8-10 is basically the same thing as 30-16-20, it's just a matter of dilution.
If you're planing on dumping a bunch of time release on them you'd want something more like the 20-20-20 to cover all growth stages.
All that said; you'd be much better off in the long run if you used organic fertz, like composted manure. Chem fertz work great for a few years, but they f$%^ up your soil after years of use.