"How is this seeding used to select the flight circle groups? "
There are four groups: A, B, C, D. Top Open seed entered goes to A, the next seed to B, then C, D, D, C, B, A, A, B, C... Then the randomly ranked unseeded Open guys, then the seeded Advanced guys, then the randomly ranked unseeded Advanced guys. When everybody is assigned to the four letter groups, the groups are assigned to circles by random draw. The whole thing happens before your eyes at the pilots' meeting during appearance judging. The folding was a nuisance for the amateur programmer.
"Does this mean that 66 people have placed in the top 20 in the last 10 years, or were there more than 66 and you just published the "top 66"."
I assume that was a question. It includes Advanced. If PW entered the data correctly, 66 people have placed in the top 20 in the last 10 years in either Open or Advanced.