As the title says, how many? Solitaire don't count.
Depending on work I play about 20 per year, about every other week. I consider that good, but seems to pale in comparison to a bunch. Don't know how they have the time.
I've played a fair amount of SK over the years, but the number of scenarios is somewhat lmited. but it might only amount to 6 scenarios a year, one eveyr couple months.