Generally, it depends on which services are running on the server. All the default ports which can be used by Plesk/services are listed below:
#20 ftp-data
#21 ftp
#22 ssh
#25 smtp
#53 dns (TCP and UDP)
#80 http (web server and Plesk updater)
#106 poppassd (for localhost only)
#110 pop3
#113 auth
#143 imap
#443 https
#465 smtps
#587 mail message submission
#990 ftps
#993 imaps
#995 pop3s
#3306 mysql
#5224 (outgoing connections only) plesk-license-update
#5432 postgres
#8443 plesk-https
#8880 plesk-http
#9080 tomcat
