Turtle tanks come in all shapes and sizes, so there is no one-size-fits-all answer to this question. However, a good rule of thumb is to have at least 10 gallons of water per turtle. So, if you have two turtles, your tank should be at least 20 gallons.
Another thing to keep in mind is that turtles are messy creatures. They will often defecate in their water, so you will need to clean it more frequently than you would with other pets. A good filtration system is essential for keeping the water clean and preventing the spread of disease.