Yes, it is coincidence that there are 18 booked and 18 names on the list. No coincidence that there are 18 booked + 2 available equal 20 - the total number of F seats. Seats booked + seats available are the total number of seats.

Names on the list without a checkmark are those still waiting to be upgraded - your place on the list is the number that is next to it. Note this changes often - since it only includes those checked in, as more people check in, the list will likely grow as you get closer to flight time. Alternatively, if those on the list cancel reservations or move flights, they'll be removed from the list.

As for those on the list with checks on it, these are those who have checked in and have already been upgraded. IME, for domestic flights, I will also be on it if I've received a CPU ahead of check in, and have selected a seat (but not if I haven't selected a seat). I believe others have speculated that on domestic flights, those using instruments who have cleared will also appear on the list as upgraded. Some buy-ups also might be added on that list as upgraded - believe those who buy -up fares.

If 18 are booked, and 12 checkmarked, the remaining 5 in F either bought an F or A fare, or were upgraded via CPU and have not selected seats yet.

From your language, I'm guessing there are 18 people on the list, including 12 checkmarked, though not completely sure. Near the top, there is a list total (or some language like that) number, is that 18? Either way, not everyone can be upgraded. Assuming there are 18 booked, plus an additional 5 waiting (18 on list - 12 already upgraded), and only 2 seats available, at the moment, there would be only room for 2 upgrades. This can change either way - if someone else buys up, someone upgraded/confirmed in F cancels, etc.