Great answers from Carol and Michelle!
One point to make is that Marketo counts Known Leads with an email:
http://community.marketo.com/MarketoIdeaDetail?id=08750000000IhFKAA0
So Anonymous Leads won't be counted. That being said, you should setup a semi-annual process to archive or delete bad leads, which can include leads that would be most likely to be suppressed, such as Invalid, Hard Bounced, Inactive.