You are reading this article to understand how many external email accounts you can add to an email alias account.
There is a limit of 4 external accounts for an alias, but unlimited "internal" accounts for the same domain.
EG: firstname.lastname@example.org can be forwarded to an unlimited number of "internal" accounts on the yourdomain.com account. email@example.com etc.
For external addresses its a limit of 4 accounts.
EG: firstname.lastname@example.org could be forwarded to email@example.com, gladisgmail.com, firstname.lastname@example.org and email@example.com.
Why is there a limit on external addresses?
When you forward an email to an external address, it counts as an incoming mail on the recipient mail server and if you have more than 5 external accounts to the same provider, for example, Gmail.com, they will block the email for "excessive" usage.