Hi M. - You're going to need at least a dozen - you might consider using the "pretty" ones as a reward. My son would pee on purpose just so he could wear a different pair of the character ones. :@ I had to make it so that he got to wear a character pair first thing in the morning but then if he got those wet or dirty, he had to wear plain ones the rest of the day. Make sure to rinse soiled underwear thoroughly before putting them in the laundry basket.
It might feel different with little girl underwear, but my sis gave me a bunch of pairs of her son's first underwear - there had to have been at least 18 pairs of sizes between 2-3. I just laundered them in hot water with bleach and tossed out the ones that weren't in good shape.
Also, if you want character underwear, definitely shop at Wal-mart. Their prices are significantly better than Target or Kohls, etc.