By my count, there are 93 SK scenarios from MMP, and a few other scenarios from other publishers. MMP just released a bonus pack so they are continuing to support SK.
The SK game system definitely stands on its own. I like to say it is 80 percent of the fun of the full game requiring only 50 percent of the rules. Those aren't exact, numbers but you get the idea. I know plenty of people who only intend to play SK. If they play one scenario a month, that's something like 8 years. I also know some players who only want to play infantry-only scenarios. More power to them.
The big concepts missing from SK are bypass movement, snipers, multi-level buildings, and concealment in all scenarios (in SK, concealment is only in PTO). There are some other important things missing, such as MG fire lanes, berserk units, heroes, deployment, and lots of other rules that are used less often. Most of the key concepts are in SK, such as the phases (including defensive fire), the to hit process, and breaking, routing, and rallying. Moving from SK to the full game isn't too hard. IMO the difficulty is learning a new set of exceptions, but that's the way ASL is.
By my count, I've played at least 1/3 of the SK scenarios face to face, and 1/2 if not 2/3 of the scenarios solo. When I first started I had played all the SK scenarios from SK 1-3 at least solo, but I haven't kept up with all the expansion and bonus packs. There are some very good scenarios in SK that have stood the test of time.