Limitations of iCloud+ own domain

What are the limitations of iCloud+ own domain in regard to emails? > ### Use Custom Email Domain with iCloud Mail > https://support.apple.com/en-us/HT212514 I am looking for: * a single domain which I already own * 3 email boxes for 3 family members (me, partner, kid) * two emails that is forwarded somewhere else (grandparents use a different email but share our domain, could be 2 extra email boxes if forwarding without an email box is not supported) * 15 aliases for my own email (I have many aliases yeah) Everything looks straightforward (MX, TXT, setup) but I need to know in advance before I commit to the change. Google Apps free tier is closing and I am not interested in buying Google's full package - I only use Gmail and YouTube. I read that there might be limitation of 5 domains (okay), 3 accounts (are these email boxes?) and 3 aliases (what?). That is really weird. Thanks!