After looking into this much longer than a sane person should have, I've realized that the strength of the tea doesn't depend so much on the quantity of tea. Controlling the strength of tea is normally done by controlling the steeping time. As stated by paul, the best way to dilute the strength of a tea is by resteeping it.
The steeping time of a tea is most determined by the coarseness of the tea leaves. Similar to garlic, finer leaves make the taste a lot stronger, whereas large, coarse leaves will be weaker. The instructions on the bag often say what's been tried and tested.
Quite often, one tea bag can make quite a lot of tea, enough for a teapot. I'd recommend going for two tea bags only if you're planning to go for larger than a teapot, for tea suited to drinking by itself and with added ice, or with certain weaker teas.
For black tea, the common steeping seems to be 2-3 minutes. There's little harm in steeping too short, but steeping too long will create a harsh bitter taste (though some people may actually want that). For cheap, harsh, unbranded teas, 3 minutes seems to be enough unless the leaves are finely chopped.
Also, as bonus knowledge, steeping tea for 45 seconds then throwing away the water and resteeping it is a great way to decaffeinate tea. Don't shake the tea bag or stir the drink while steeping it, because it will cause the harsh stuff to enter water faster, and black tea should be steeped at 98 degrees C.