Most children switch from two naps to one between the ages of 12 months and 24 months. When a baby should start napping only once a day depends largely on the baby, the parents' schedule and the environment.
Cutting a baby’s daily routine down to two naps from one is not about what the child thinks he wants or the schedule the parent would like to have; it is about the biological need for two naps versus one. Naps at different times of the day serve different purposes in mind and body development at different ages; for example, morning naps have more dreaming (REM sleep), making them important for young babies who need this for early brain development. The best way to tell if a baby can cut down naps is to do trial and error to see what works.