Earlier quoted context omitted.
They can know how many people, as long as they don't store personal information. Keeping a counter doesn't require consent.
Keeping a reliable counter of meaningful interactions is hard without personal data to correlate unique users.
It's hard if visitor numbers would directly translate into revenue or similar. The trouble you're running in then is discerning organic hits from clickfarming.
If all you care about is when, where what content is popular on your website, there's a pretty simple method: Tally 200 and 304 responses. 200 tells you how many visits you get. 304 tells you, how often people hit refresh, or re-visit your page within the expiration time of the URL that 304s.
Also there's little value in identifying individual visitors. Getting a coarse idea where visitors are located in the world might be nice (for a regional news outlet for example). So just slap some coarse grained. GeoIP on it.