anyhow if you give them revenue like upgrade money or miles, those seats should be yours, their allowance should be "F space available".
i recently bought 3 out of 6 F seats on a 777, and the remaining seats all got used during the flight (did not bother us as we had the left side of F class to us). on the right side, the purser moved his office to 1K, and other crew members used the remaining seats during the flight.