Edit file /etc/dovecot/conf.d/10-master.conf and add directive process_limit with the amount of processes you want to use (by default the limit is 100) to service imap-login.
For example, if you want to raise the limit to 200:
service imap-login {
inet_listener imap {
}
inet_listener imaps {
}
process_limit = 200
}
Don’t forget to restart dovecot after the change:
systemctl restart dovecot