Literally, millions. There were made by Russia, China, and many of the Soviet allies. While more AK47s have been made, there are a LOT of SKS rifles out there. No exact number are available.
Some SKS rifles were brought back... most of them were manufactured in East Germany or China, as the Vietnamese made SKS rifles were and are extremely rare. They were either captured or sold/traded for favours to other soldiers in-country.
7.62x39mm, which is used in all SKS rifles (aside from the former Soviet Union, the SKS was also manufactured in China, Vietnam, North Korea, Albania, the former Yugoslavia, East Germany, and Romania).
First off, you didn't specify which army, nor did you specify the country of manufacture - the SKS was manufactured in the former Soviet Union, Romania, East Germany, North Korea, Vietnam, the Peoples' Republic of China, the former Yugoslavia, and Albania. A lot more information would be needed in order to determine this. Bear in mind that not all SKS rifles manufactured saw military service - many of the M59/66 rifles were never issued, and China continues to manufacture the SKS for the commercial civilian market.
No country which manufactured the SKS gave it the nomenclature of M21. The only country which applied an "M" number which I can think of offhand was Yugoslavia, who designated theirs M59, and M59/66 for rifles which had the 22mm grenade launcher fitting. Yugo SKS rifles can go from $150 - $500, with the latter price being for an unfired, unissued rifle. SKS rifles in general can go from $100 - $1300, depending on the exact model and country of manufacture (Vietnamese and North Korean rifles have the highest value, being the most rare). You can send photos of the rifle and markings to my email (listed on my profile page), and we can determine the exact model and value from there, if you wish.
Good question. Unfortunately, we don't have an answer- they came from several nations at different times. One estimate is that there are between 5-7 MILLION SKS carbines in the US.
First off, the SKS isn't an assault rifle. Second off, they were manufactured in several countries. Dependent on origins, whether or not the numbers are all matching, condition, etc., you could be looking at anywhere from $200 to over $1000, with $1000 being geared more towards extremely rare ones, such as the North Korean or Vietnamese manufactured rifles.
Seems to work well for all firearms.
The precise number of Winchester Model 1894 rifles that were manufactured is unknown, however, the number is around 7.5 million. This rifle was designed by John Browning.
you can find sks rifle at virtually any gun shop but searching for a Chinese version may be a little more difficult.
In the vicinity of $200 for a basic Type 56 carbine, up to $500 - $600 for some of the more 'exotic' models, like the paratrooper carbine, and detachable magazine SKS D and SKS M rifles.
In the USA
The Chinese manufactured several variants of the SKS. Depending on specifics, you may looking at anywhere from $200 - $600+. Paratrooper models and the detachable magazine SKS-M and SKS-D fetch much higher prices than a standard Type 56 carbine.