Generally, when harvesting herbs, you can take a few leaves at a time, or if you would like more, cut off about 1/3 of the branches when the plant reaches at least 6-8 inches tall.
By cutting close to a leaf intersection, the plant will regrow quickly.
Some plants, such as parsley, grow new leaves from their middle. In this case the oldest branches need to be removed, leaving the new tiny branches growing from the center.
For more details the Old Farmer’s Almanac is a great resource: