The number of matches that count will very depending on how many teams are there. Smaller regionals you might get 10-11 matches, while bigger one you might get 8-9 matches. For picking for the elimination rounds it goes 1-8 and then 8-1. The #1 seed can pick anyone they want. If they picked #2-#8 seeded teams the team selected has the option to decline.
Example: Team #1 picks team #3. Team #3 says no to team #1, with the chance to pick for themselves, and team #1 picks someone else. Team #2 cannot pick team #3.
Now if some one out side the top 8 says no, and doesn't move into the top 8, they can't say yes to anyone else.