Visitor count is the number of users who have visited the website.
If a visitor visits twice in the selected date range, it will be considered as one visitor.
One person can use a device from a different location (home and work) and be counted as two visitors
If there are many users using one device, it will only count as a single visitor (an example is a family using a desktop at home)
If a user clears their cookies, it may be considered as two separate visitors
To read about the difference between visitors and visit count, you can check out "What is the difference between Total Visits and Visitor Count?"