as far as college basketball goes you are correct. The rule has always been that the official needs to be able to call the number on one hand. So the 55 would be the highest possible number. So basically it can be the following:
0-5
10-15
20-25
30-35
40-45
50-55
The NBA used to be like this, but in recent years has allowed crazy numbers including 99.
So in reality College basketball teams only has 36 available numbers for their use.