Each hornworm comes from eggs laid by the Hornworm (or Tomatoworm) moth. The moth tends to be relatively large and brown. How many hornworms you get depends on how long the moths visited your garden before moving on to greener pastures, so to speak.

As I said in a previous post, go out to your garden at various times of the day and try listening for the hornworms eating. It sounds kind of like a medium-low pitched "crunch. . . crunch. . . crunch." If you hear them it can help you track them down.