Every time I've ridden the bus bridge, regardless of whether it's a weekday or weekend, morning, afternoon, or evening, it's always been three buses. That's why the popular trains such as 774/777 and 784/785 frequently have a sold out bus bridge. I did not ride the weeks of Thanksgiving or Christmas, so I do not know if they added capacity for the holidays.
The bus bridge is a reserved service, so they do not oversell it. They do check each person's ticket before boarding the bus.
If you have a train-only ticket for a date that's been converted to a bus bridge, call Amtrak immediately to have your ticket reissued. It is not automatic. Do not wait until the day of travel because you want to make sure you get on the bus before it sells out.
One of the few pluses of the bus bridge is that the trains won't be standing-room only, and north-bound passengers getting on in Orange County can actually find empty seats.