Do people who compete in Silhouette matches generally compete in only one class or in more than one?

Most of the Sil. shooters I know compete in multiple classes. The classes they choose depend on their interests and abilities or disabilities.

The CLASS is determined by the shooter's scores, so changes if he/she improves.
Are you asking about the CATEGORIES OF COMPETITION?
B. Big Bore Competition




1. Competition fired on full scale targets consisting of three Production handgun categories and three Unlimited handgun categories:




Production (P) Unlimited (U)


Revolver (R) Unlimited Standing (US)


Standing (S) Unlimited Any Sights (UAS)




2. Competition fired on half scale targets consisting of two Unlimited handgun categories:




Unlimited Half-Scale (UHS) Unlimited Any Sights Half-Scale (UASHS)






The letters in parentheses are the official abbreviations for each category (typical below).




C. Small Bore Competition




1. Competition fired on three-eighth scale targets consisting of three Production handgun categories and three Unlimited handgun categories:




Production (P) Unlimited (U)


Revolver (R) Unlimited Standing (US)


Standing (S) Unlimited Any Sights (UAS)


Yes, the same gun can be used in more than one category, which is not uncommon.

__________________
Science plus Art

Yes, I mean categories.

I meant categories too.

Thanks much. Trying to figure out how many to take to my first one.

With a .22 revolver, you can shoot one firearm in multiple categories, even if it's not theoretically ideal.
For instance, with iron sights you can shoot:
Revolver (An obvious match but you compete against lots of $2000+ Freedom Arms revolvers. Not a big deal unless you worry about taking home 1st place consistently.)
Standing (Perhaps the best category, in which it's more about the shooter than the firearm.)
Production (Great for warming up/sighting in, if needed)
Unlimited
1/5 scale (A real challenge, though!)
And you can shoot the optics categories as well with iron sights, but only if you are a glutton for punishment!

With a centerfire iron sighted revolver you can not only shoot the Big Bore categories similar to those Smallbore variations above, but also Field Pistol (1/2 scale targets shot standing at Smallbore distances. Very difficult, but rewarding!)

If you like Optics on your centerfire revolver, probably the best categories in which to compete are:
Field Pistol Any Sight (100yd)
Unlimited Standing (200m)
Otherwise you are restricted to Unlimited Any Sight and 1/2 scale UAS. Some ranges also include Hunter Pistol. (There's both a Short Course and a Long Course version. 6x max magnification shot freestyle on both.)