From what I understand, in the early days of SASS, the shooters used a single revolver. That means either getting only five pistol rounds downrange on each stage or you had to do an "on the clock" pistol reload. Over time the game evolved to employ a second revolver. It means more shooting while keeping the match moving.
My home club is pretty typical in that we shoot six stages in a monthly match. If we used a single revolver, that would be only 30 pistol rounds downrange. Hey, I came to shoot!
True, SASS does not have an official category for the use of less than four guns. It would be hard to incorporate that since in addition to category awards most matches include awards for overall placement and since we're a timed event, a category that involves fewer firearms (fewer targets per stage) would give shooters in those categories an unfair advantage, or else they'd have to exclude those shooters from eligibility for overall awards.
Local clubs have the flexibility to create "unofficial" categories and I've heard of some clubs recognizing categories with fewer than four guns. I've also heard of local matches that allow adults to shoot .22 caliber. That may be available at your local club if you ask.
It is very common in my club for new shooters to have less than four guns when they start out. In fact, we encourage folks to come out before they buy guns. If a shooter has a single revolver, the SASS rules allow for two possibilities: take the penalties for five unfired rounds on each stage, or reload the revolver "on the clock".
In five years of shooting I have never seen either situation. This is because if you show up to shoot a match with one revolver, just about every other shooter on the posse will lend you a second revolver. This is not in the rules but it's in the culture. The clubs in my area are eager to help new shooters and this is one way that we do it. Personally, I had zero guns when I got started so I shared guns for a year before I had all of my own stuff.
SASS does not claim to be 100% authentic. I know some SASS shooters who enjoy using period correct gear (right down to the ammo boxes) and if that's what you enjoy, great. If you want to shoot a match where the rules require this of everyone, you might really enjoy getting involved in NCOWS.
Yadkin, sorry to hear your report. I don't think that is representative of SASS clubs. The clubs in my area are family friendly and I have not seen alcohol on the premises during or after a match. Some of the clubs in my area shoot on Saturday, which is my preference, and some shoot on Sunday (in my local club all of the matches of any type are held on Sunday). We have cowboy shooters who don't shoot matches on Sunday and I respect that. I also know of a couple who own their own business that is open on Saturdays, and the Sunday matches allow them to participate.