Is the prefix web#_ required for each username?
No, but recommended. Email users must be unique for the whole server, so if you do not add a prefix, you have to make your own naming conventions. Additionally if you run postfix and use config files in sendmail style, the system will accept email for system usernames directly, so if you create a user "info", this suer will get email sent to email@example.com
on your server, if firstname.lastname@example.org
is not a regular mailbox created under this domain.