From one single seed. Selection will usually determine that only one plant survives, but if more than one does, there will be a limit such that the clumps don't fully develop.

Overseed too much and they choke each other out, compete, and--as always in nature--the strongest or luckiest survive and you get the clumps again. They may not fully develop in good conditions again, of course.

So everything depends on treatment, time, and so on.