Hi Kali,
you could just use the Accounting options like this:
AccountingMax 1 TB AccountingStart month 1 0:00 (<- this is default)
This will cause your relay to suspend until the end of the month after forwarding 1 TB of traffic. You might want to set the limit to 950GB instead.
Another option, if you want to avoid possible suspension at the end of the month, is using the RelayBandwidthRate and RelayBandwidthBurst options to limit the forwarding bandwidth.
Further explanation of the options can be found here: https://www.torproject.org/docs/tor-manual.html.en
Regards, Sven.