I’m having a hard time understanding how to calculate buffer capacity (K) when analyzing a queue

I'm having a hard time understanding how to calculate buffer capacity (K) when analyzing a queue.

For example, I understand that if you work for a call center and they have 5 lines, that means that the buffer capacity is 4 since the CSR will be on one line and 4 can be waiting. If a 5th calls, they wouldn't be able to join the queue.

However, I'm having difficulty translating that into a different scenario. For example, if you are going to a baseball game and there is 1 scanner at the entrance with a queue that designed to hold up to 50 people waiting before getting to the ticket scanners.. Is the buffer capacity 49, 50, or 51?

I can't seem to wrap my head around it…