Yahoo! Small Business accounts may include up to 1000 email addresses depending on the type of account. To see how many email addresses you have, visit the Email Control Panel and click "Additional Options".
Email Control Panel Additional Options Link
In the Email Summary module on the right side of the page, you'll see the total number of addresses available in your plan.
Email Control Panel Additional Options Email Summary
Note: Because a Custom Mailbox plan includes only one email address, Custom Mailbox owners cannot create more email addresses. If you'd like more addresses, upgrade to Business Mail, Web Hosting, or Merchant Solutions.
Learn how to upgrade if you need more email addresses.