Personally, I think it's the wrong question. I care less about which tenants occupy our commercial buildings than the form those buildings take. Are the buildings compatible with complete streets? Then, let a thousand Starbucks bloom. Are they incompatible? Then, keep 'em out. Because tenants come and go, but the form a city takes will last decades and even centuries. Get it wrong now and our grandchildren will be living with the consequences long after we're gone. I regretted the planned drive-through-only Starbucks in the Richardson Heights shopping center for that reason, not because I personally will never shop there.
Others want more variety in Richardson than a proliferation of Starbucks is leading to. That's an acceptable attitude. Others might want to limit Starbucks because it's a national chain that crowds out local, better coffee shops. That, too is an acceptable attitude. Others might object to that drive-through-only Starbucks in the shopping center because of the effect it will have on traffic on that corner. Similar worries led the City Council to regulate big box retailers in Richardson. Still others might even see Starbucks as a public nuisance catering to the public's addiction to caffeine. That attitude led others to argue against hookah or electronic cigarettes or even karaoke. These arguments carried the day with the city council, which regulated these businesses.
As you can see, there are many factors that have to be considered before you can answer the question, "How many Starbucks are enough?" It isn't as simple as waving the flag of free enterprise and saying the answer is up to Starbucks and Starbucks alone. Unless you're willing to give the same answer when someone asks, "How many hookah lounges are enough?"
But, again, I argue that it's the wrong question to be asking in either case. The right question is, "What form do we want that Starbucks (or hookah or whatever) to take?" Because that's what we'll be living with long after Starbucks is gone.