By defining the Single Message as below, you can send only one email at a time. If you want to send 10 emails, it is 10 instances of the single message.

> Messaging.SingleEmailMessage mail = new
> Messaging.SingleEmailMessage();

Change your definition as follows.

> List< Messaging.SingleEmailMessage > mail = new List<
> Messaging.SingleEmailMessage >();

By doing so, you can add 100 members to the list. All the 100 emails are sent with the method Messaging.SendEmail(mail);