We know of at least four incarnations:
- The baby Melody Pond ("A Good Man Goes to War")
- The young girl (alias unknown) "The Impossible Astronaut"/"Day of the Moon". Dialogue prior to her regeneration in "Day of the Moon" indicates she has regenerated before.
- The teenage-early 20s incarnation known as Mels ("Let's Kill Hitler")
- River Song
We know River Song is the last incarnation as she loses her regenerative ability soon after that incarnation is born, and we see her die in "Forest of the Dead", though it could be argued that her data clone version housed in the CAL supercomputer counts as an additional incarnation. We do not know if she regenerated between her first and "young girl" incarnations (as noted, she is perfectly aware of regeneration at the time of her change, meaning she's done it before). We do not see her change into Mels so it is not known how many incarnations may have existed in-between. Given that "young girl" regenerates decades before Mels becomes Amy and Rory's friend, other incarnations are possible, however they have never been chronicled.