I find some result.
It seems that It shows some solution. But I can't understand what does it mean
Linux+ Study GUide document by Roderick W.Smith
It says like below
"(The limit is 65,536 users with the 2.2.x kernels and over 4.2 billion with the 2.4.x kernels, including root and other system accounts. The limit can be set lower in configuration files or because of limits in support programs.) "