Every GreenGeeks hosting plan except Lite50 includes unlimited email accounts (webmail, IMAP, POP3). Lite50 is limited to 50 email accounts. Newly created email accounts have a 250 MB storage quota by default.
250 MB of space is sufficient for many email accounts, but if you run into any problems with an account using the default value, the quota can be increased, decreased, or set to unlimited in cPanel.