The 9th Circuit Annual Report for 2018 has some pretty interesting statistics. One is that there were 955 petitions for rehearing en banc last year. Of those, 17 were “called” for a vote. Of those, 8 petitions were granted and 9 denied. So 8 out of 955.
The way it works is that the petition for rehearing en banc is sent to all 27 “regular” or “active” 9th Circuit judges. The “senior” status judges do not get to vote. Any one of the 27 regular status judges can call for a vote. If none makes the call, the petition is denied. Once a judge makes the call, the 27 vote for rehearing and it takes a majority to grant the petition. I’m not sure of the timing, i.e., how long it is before the petition is denied because there was no call.
The en banc panel is 11 judges consisting of the Chief Judge and 10 other judges chosen at random. It hears oral argument and rules. It affirms or reverses the three judge panel that it is reviewing. A nice summary of the rules is here.
Why en banc? I think sometimes there is a sense that the three judge panel got it wrong. But more often and usually, the three judge panel was bound by a prior 9th Circuit ruling that was wrong or needed to be better explained or modified.