Hi
sorry it seems I missed this message.
Amazon charges based on hours of usage, not based on number of instances you are running. Check this Amazon Free tier limits on Amazon Website.

As far as I know the free tier covers 750 machine hours of micro instances. You can probably run 750 machines for 1 hour and finish that quota in 1 hour. Or run 1 micro instance 24x7 and it will stay up for a whole month.

But note that AWS might suspend your account if you try to abuse their system.