I want to know if there is any way to know that how many messages I have sent from my Airtel Mobile in a day because I am using 21 sms pack per month that provide 100 free messages per day. Due to this every day I lose some of my mobile balance unknowingly.

Well, there can be few tricks, some phone comes with message counter which keeps track how many messages you sent and received. You can daily see the counter and when 100 more messages are added, then you have crossed your limit. Other way is to daily clean up your sent SMS folder and set it to store upto 100 messages, when it fills up, then you know that you are about to cross your sms limit. Though above things depends upon your phone model.