pj-ftp-server
Pure Java FTP server. Free portable cross-platform multi-threaded.
... support
12) Support limit max concurrent logins
13) Support limit max concurrent logins per IP
14) Need JRE-1.8:
https://www.oracle.com/java/technologies/javase/javase8-archive-downloads.html
Source = https://github.com/harp077/pj-ftp-server
If you like this - put a star on https://github.com/harp077/pj-ftp-server/ for support project please.