One FTP user can never manage more then one website in ispconfoig as every website has its own Linux user for security resaons and one FTP user is always assigned to one Linux user by the ftp daemon.
It would be silly to create 20+ FTP accounts for one client.
Not at all, if you take in account that it happens from time to time that a website gets hacked. It makes a bit difference if you would have to clean 20 hacked sites then instead of having to clean just one site. Every FTP application is able to save FTP accounts, so there should be no problem to save one account for every website, especially as every website is most likely in a separate directory anyway.