IMO for maximum number of daily visitors, there is no limit and for minimum you should have an average minimum traffic of 2000 per day for a starting handsome income from ADs, every month.
I agree to Davidlux, more visitors more money you will get thus you need to increase your traffic daily by promoting your sites on other forums, blogs or websites. Also need good backlinks it will help your site has high google PR and you will attract your customers to.
You can make money with little traffic, it depends on the method you are using to monetize your site. But if you choose g-adsense or other PPC as a way of monitizing, then you should be looking at, at least 1000 unique visitors daily.