If the maximum turtle quantity shows 0, then your tank is too small to keep even one turtle of the mentioned shell length.
This calculator works on the following principle:
A commonly accepted rule is that a turtle requires at least 10 gallons of tank volume for every inch of its shell length. For instance, a 4-inch turtle would need a 40-gallon tank, while a 12-inch turtle would require a 120-gallon tank.
So, if you are planning to keep baby turtles (2 inches shell length) and your aquarium holds 50 US gallons water, then you can keep 2 of those turtles.