One is all they need if it emerges alive, and returns from the mating flight...
Bees may initiate swarming, but do not "usually" gather if they have no queen.. When they fly with a queen, she is NOT an exceptional flyer, and depending on where they are headed, my pause and gather along the way while the queen rests.
When there is no queen, they will often take to the air, fly off a short distance, and then begin returning. I have watched this happen with a "clipped" queen, meaning her wing was snipped so she could not fly.. the bees launched, the queen launched.. flew half a dozen feet and ended up in the grass.. the bees gathered around her for a time, and eventually returned to the hive, leaving her with about a dozen bees... They waited for the first queen cell to emerge, and then swarmed with the virgin queen.
Come to think of it, Dr. Seelys (Spelling?) videos that Mrs. River posted shows this as well, the bees flying off and then returning because the queen was actually caged.
Excellent videos, I have never heard the piper scouts, or witnessed the Buzz runners.
http://www.worldwidebeekeeping.com/forum/index.php/topic,5770.0.html