Xero accounts on a Trial or subscribed to the Xero Starter plan have a limit on the number of invoices.
If you are on trial, you can use their Demo account for testing as this does not have a limit on number of invoices. If you are ready to proceed with Xero you will need to subscribe to a plan that includes the number of invoices you require, the starter plan is the only plan that includes a limit on number of invoices per month.
If you are subscribed to a Starter Plan, you will need to upgrade your plan( http://www.xero.com/pricing/).