Will I get less spam?

Over time, yes. When you get spam sent to any email address you have given out, you can block that address. You also know who is responsible for selling or losing that email address.

How many email addresses can I have?

The number is unlimited (except by your imagination). Give a different email address to everyone you correspond with if you like. anything@you.emptyjar.net will be sent to you, so you can give that friend who likes to forward chain emails, the company wanting to send you an email reciept, and that annoying salesman their own special addresses.