Infants under the age of 2 traveling domestically on your lap don’t require a ticket and can fly free of charge with most airlines. However, you must warn the airline that you’re going to fly with a baby. You can add your baby to the ticket after booking it by contacting the seller.
Airlines also have a rule that only one infant without a seat can fly per adult. If you’re flying alone with two infants, you’ll need to buy a child ticket with a separate seat for one of them.