I’d say it all boils down to what type of pet site you’re planning to build. If you’re going for a more traditional pet site—something in the realm of Neopets, Subeta, Marapets, or Powerpets, then starting with 10 to 30 pets is a solid range.
However, if your site will feature exploration mechanics and pet customization, you’ll probably want a wider variety of pets available at launch to give players more options and replayability.
Especially if users will be exploring areas and capturing pets, variety becomes even more important. These days, some users may expect more than just 5–10 basic options; a larger roster of pet species tends to attract more interest and allows players to connect with something that fits their style.
So overall, I’d say aiming for 10 to 20 pets at launch is a smart, balanced starting point.