Setting min-users too low causes the server to have to create and destroy processes too frequently, which is a system-intensive operation. Setting it too high may leave ncftpd processes sitting around that never activate, but it is better to overestimate rather than underestimate because those idle processes are just that -- idle. They don't consume CPU time or resources when they aren't active. The memory the idle children use will be reclaimed when the processes are swapped out by your OS.
Examples:
