Is there any limit for sending mail from a developer org in salesforce?

Hi,

Yes there is a limit of 10/day to use single email method to send emails. But you can use it to send more then 10 email/day for that you need to attachemed more then person email id and then use send email method of single email class to send email in bulk.

The limit is to use sendEmail method 10 times in a day not to send only 10 email/day. To send more then 10 email you need to create a logic which send email to more then one person by using single call of sendEmail  method by adding email it to setToAddresses[] method, setCcAddresses[] ,etc .

Thanks.