Sorry for being a bit dim but can you explain that?
Since people are turning up essentially at random, sometimes (actually a lot of the time) there will be empty spaces, while at other times there will be a queue even though there would have been enough capacity for those people if they'd turned up slightly earlier or later.
This subject has been extensively studied mathematically ( Queueing theory - Wikipedia ), most particularly for telephone systems - how many lines do you need to carry a certain amount of telephone traffic? This is very close to the same question as "how many supercharger stalls do you need to serve a certain amount of Tesla traffic".
For telephones, the numbers are normally stated in terms of how much traffic can be carried by a given number of lines for a certain quality of service - where quality of service means the probability that you won't be able to make your call because all the lines are in use.
So for a quality of service of 0.01 (1% chance of finding all lines occupied)
1 line can carry 0.0101 units of traffic
2 lines can carry 0.1526 (15 times as much as 1 line)
4 lines can carry 0.8694 (5.7 times as much as 2 lines)
8 lines can carry 3,128 (3.6 times as much as 4 lines)
12 lines can carry 6.615
90 lines can carry 74.86
If you are prepared to accept a poor quality of service, say 10% chance of finding all lines occupied, things get more even but still capacity climbing faster than number of stalls.
1 line can carry 0.111
2 lines can carry 0.595 (5.3 times as much as 1 line)
4 lines can carry 2.045 (4.4 times as much as 2 lines)
8 lines can carry 5.597 (2.7 times as much as 4 lines)
12 lines can carry 9,474
Intuitively you can see that it would be like this - you always need to keep a couple of stalls free to handle new arrivals that might or might not turn up, but the more stalls/lines you have, the more of them can be working away solidly rather than kept in reserve.
The Supercharger numbers won't be quite the same as for telephones - there's factors like the sharing of cabinets between pairs of stalls, the time taken to enter/exit stalls, the behaviour of drrivers when all stalls are taken etc. I expect Tesla has their own numbers that are more accurate, but it's going to look something like this.