Tell me!! If you say only one, I’ll say you have more than one.
Lets begin from here. Be your gmail login ID be firstname.lastname@example.org for sometime. Compose a mail to email@example.com or firstname.lastname@example.org or anything which varies by ‘dots’.
What was the result?
You’ve just sent a mail to self. If a person registers with gmail as some email@example.com, then from then any variation to that username including capitals,dots will be unavailable for use.
In this case, ABCXYZ@gmail.com or firstname.lastname@example.org etc., will be unavailable to others to register.
This is just another effort from Google similar to the googlemail.com domain to combat spam.
But the other side of this is that, this may also increase your spam numbers if your username is smaller. People using the dictionary listing to send spams will generate random username’s and forward an email to that generated email. And since any email with a variation of your username will only be sent to your inbox, you may find plenty of spams should your username is very small and easy to guess.