This is exactly why they started charging for Supercharging because some were abusing it. Keep in mind that even though they're charging for the use of the network they're profiting from it. They're also using the money coming in from that to dump right back into the network and expand it and improve it. In 2016 Tesla announced they would double the number of Supercharger stations in existence in 2017 & 2018 which they did and then some. They announced at the end of last year that they would double the total number again but this time they'd do it in a single year which would require four times the rate of installation. This is in answer to the massive ramp up in sales to make sure that this doesn't become an issue. Increasing the charge rate will also do a lot to free up the stalls.
They also started enacting fines for those who remain in stalls beyond their battery being full. I personally have never heard of anyone actually being charged for this but they sure warn you of it's existence.
To answer your other question, there is usually about 6-8 stalls with some being in the dozen or more range. I've never experienced a situation where I pulled up to a stall and didn't have one available for use. In fact, the nav shows you how many stalls each location has as well as how many are currently in use so you can choose to go to another one if it's full. This is super rare other than highly populated areas and they usually install several nearby for this exact reason.
While I agree that waiting in line would be terrible I can tell you from personal experience that there would have to be a TON more cars using them for this to be an issue. Most of the time we pulled up to a station that had 10 stalls and we were the only one there the entire time we were charging. That's not to say that some weren't busier but it was the exception and not the rule. In short, while it is possible, I'm not too worried about it.

