How much dried lemon zest to substitute for "zest of one lemon"?
Use one teaspoon of dried zest for one tablespoon of fresh zest, which is about what you get from a lemon.

Most foods dry to about a third their original size, so I would use one third of the amount called for. Lemons vary a great deal in size, but the recipe probably means one tablespoon of fresh zest, which is normal for "medium" sized lemons.

Something to watch out for is that most cheap "dried lemon peel" products are actually made from the zest and a bit of the pith, which will both give you a bitter flavor and short you some of the lemon flavor you expect. Look closely at the zest you buy, if you can see that the individual pieces have two distinct layers, or a pale yellow or white color, steer clear of that brand. This is probably a "get what you pay for" situation, so spring for the higher end lemon zest.

I have also had great luck drying my own lemon zest. Unless your kitchen is humid, it will probably dry in a day or two just sitting on a plate on the counter.