The number of monthly visits is the sum of IP addresses seen within a 24-hour period as recorded in the Nginx logs. A few things to note:
A human lands on your site: +1 visit
They browse a new page: +0 (still the same visit)
They land on your site from a different browser: +0 (still the same IP)
The land on your site from a different device: +1 (different IP address)
There may also be bots counted as visitors, which is why your Google Analytics or Cloudflare Analytics might show slightly different numbers. This is all explained thoroughly in their excellent resource here.