How to configure or modify FTP concurrent user number?

It is related to FTP passive port range, each user need 2 passive port, please go to FTP Service to adjust the passive port range.

EX:
Default passive port range at 30000~30020 for 10 concurrent user.
If would increase to 20 concurrent user, please change to 30000~30040.

NOTE:
How to know how many concurrent users been log on FTP already?
https://thecus.kayako.com/Knowledgebase/Article/View/how-to-know-how-many-concurrent-users-been-log-on-ftp-already