Per the Governor Limits, you can only send 5000 single emails a day (or less, depending on license).
Using the API or Apex, you can send single emails to a maximum of 5,000 external email addresses per day based on Greenwich Mean Time (GMT). Single emails sent using the Salesforce application don't count toward this limit. There’s no limit on sending individual emails to contacts, leads, person accounts, and users in your organization directly from account, contact, lead, opportunity, case, campaign, or custom object pages.
http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_gov_limits.htmhttps://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_gov_limits.htm