My preference is to have 3; (12, 3 and 9 o'clock). The reason for that is because the rail can then be supported underneath along its length. Rigidity of the guide rail is very important.
I used hollow stainless tube and bolted it down from underneath through holes along its length and a piece of inserted metal plate with threaded holes at the same point as the holes in the stainless. That has worked well.
Once the trucks are in place, I then spun them by 45 degrees placing the bearings at 7:30, 10:30 and 1:30 (ignore the minute, only consider the hour). That then provides a locking method for the trucks.
If you don't spin by 45 degrees, you will have to find a way to stop the trucks from lifting. I used the 45 degree spin on the Y axis only. On the X axis, I have dropped a plate below the side of the truck with a bearing that hits the underside of the rail support.
I explored having a non supported rail through fibreglass but was advised against it (no structural strength). I was told that a better method would be to insert two plates in the shape of a cross and weld them at either end.
I opted for the stainless tube method as I argued that almost rigid is not good enough for CNC. Nothing here is based on evidence only opinion. I am still a noobie.
For me, rigidity is the most important design consideration in CNC.