It's not about there being "enough" gas stations. It's about location. If people are willing to go to a new gas station because it's in a more optimal location for them, then there's a market for it. People often prefer one station over another just to avoid left turns.

There may also be other factors at play with the convenience store. Is there a store like that within walking distance? That's another market opportunity. In my home state there is a gas station chain who is very popular for their pizza. They actually opened another station in my home town specifically to keep up with demand for the pizza.