Content is more important than visitors.
I see that to mean “if you build it, they will come” and I agree with that.
As far as how many visitors to make a living – I couldn’t tell you if you held a gun to my head.
It is entirely dependent on your visitor’s exact demographics in relation to what is offered on your site and how it is presented; and how the sent clicks convert at your program of choice and how long the specific casino can retain them etc etc..
If the formula was so simple (x visitors = x dollars) everyone would make a ton of money.