If you do not want a full book of 20, your options depend on the location:
: Many post office lobbies have kiosks that allow you to print individual stamps or purchase smaller booklets. These are available 24/7, even when the retail counter is closed.
: USPS occasionally offers smaller sheets or "panes" containing 10 or 18 stamps , which may better suit infrequent mailers than a 20-stamp book. Retailers That Require Full Books
Most major retailers only sell stamps in to maintain efficiency at checkout. You typically cannot buy single stamps or partial books at: How Many Postage Stamps are in a Book?
: This is the most reliable place to buy any quantity of stamps. You can stand in line and purchase exactly one stamp or any other irregular number.