Ask HN: How many visitors/mo needed to be successful?

Hello, I am currently founding a new website as startup/idea but of course I will go on by myself without funding..

The website is around social music.

I need to know how many visitors/month should I target within a year, to be really successful?

Because I know that a successful web app/site is associated with how many registered users + how many users.

I have the ability to reach a million visitor/monthly by the end of the year, but is it considered a success?

To be more specified, I am looking for making a startup to sell it later and get acquired. :)