So if you are planning to be a responsible and informed pet turtle owner, definitely don't impulse purchase a pet turtle. The thing you need to do is read about the species you're interested in, and find out important things about it like: How big does it get? How long does it live? What does it eat? And what sort of environment does it need to be housed properly in your home?
So for example, these red-eared sliders, which is this large turtle over here, are often sold, unfortunately, illegally as very small hatchlings. So you purchase one of these and they're about the size of a quarter. But as you can see, with our friend over there, they can get quite large. So if you can not provide a home for 50 years or more with adequate space, and the adequate enclosure set up including lighting, heat, proper diet, and a stable environment, then you should not become a turtle owner.
So if you're trying to decide how many turtles to keep, it's again important to know what their behavior is like. Some turtles are not social, and they will actually fight with their cagemates. Other turtles do well in groups. And again, this depends on the amount of space you have to house the turtles. Again, the more turtles you have, the larger the space. Also keep in mind that you should check with your state regulatory board to figure out whether you need any type of permits for keeping more than one of a certain species of turtle.