How many visitors does my site need to make money with AdSense?

There is no minimum of visitors, pageviews or hits to get approval for the AdSense program.

Therefore there is no minimum of visitors, pageviews or hits to make money with AdSense.
The only limit is zero. Because if you have zero visitors, there is nobody to click on your ads.

Every visitor to your site is a potential clicker/buyer and therefore a potential moneymaker to you.
In general, the more people visiting your website, the more money you make.

The amount of money you make per visitor differs per website-topic, type of the site, size of the site, time of day/year, etcetera.

Sites with large quantities of original quality content are likely to earn more money than for example forums.
So it is possible that a site with 10 visitors a day makes more money than a site with a 100 visitors a day.

But in general, sites that offer quality content but only have 10 visitors per day or less, don’t even make enough money to pay for the domain and hosting.

If you have a site with quality content that has about 100 visitors per day, there is a chance you earn a few cents every day and maybe sometimes a dollar or more a day.

If you have a website with quality content that is visited by about a 1.000 people per day, you are likely to make at least 1 dollar per day. If your content is durable and will not lose its value, you will make that money each and every day, year after year. An asset like that can make you thousands of dollars in the long run.

A site with quality content that is attracting 10.000 visitors a day? A site like that is very likely to be one of your main sources of income. In some cases a site like that will be able to fire your boss, so to speak.