I think it depends mainly on how long the tang is. With a short tang, as in an English sporting rifle or pelter, only one screw goes down from the top, but there is frequently another that comes up from the trigger plate ahead of the trigger guard's bow, and into the underside of the standing breech.
On a Hawken, with its long hour glass tang, two screws go down from the top into the 10 1/2" long trigger plate.
I have no experience with the long tangs of Southern rifles, but I would guess that the forward most screw is a machine screw going down and into the trigger plate, and those behind may very well be wood screws.
The screws themselves should not in my opinion, absorb the recoil of the rifle. The fit of the standing breech, or for that matter, the breech of a Kentucky type of rifle, should be fit tightly against wood and it is the stock itself that receives the energy, not the screws. and while we're at it, don't expect the barrel retaining pins or keys to soak up recoil either...it's the contact at the breech that does it all.