You are talking about aliases. I suggest some stuff you might want to read on this topic are:
How to set up aliases
Plus addressing and subdomain addressing
Identities
I would also include
Catchall/Wildcard aliases but I believe there's a error in that article (step 3) because you set these up as sending identities not as actual aliases (you cannot define an alias whose local part is a "*"). Maybe I will submit a ticket on that.
In general you can have up to 600 unique aliases but by using subdomain (or plus) addressing you can "overload" a single alias with
any number of email addresses.
I'm pretty sure others will follow up in this thread with some examples. If I recall there's another thread with some examples but I don't have the link handy.