A lot of people buy their tickets on line. If that's you, go through the app and see when you travelled.
If you don't buy online then most likely you pay with plastic. And you should know how much the tickets cost. So go through your bank/credit card statements and add up the times when you bought tickets.
If you're someone who buys your tickets with cash, then you may have to do some estimating. You've already told us that you have been doing this for 5 months (the thread title) and that you travel around 3 or 4 times a week (the original post). So unless you have a diary that gives the days when you travelled (presumably the days you were expected in your office rather than working from home) and assuming 5 months each of 4 weeks, then you have done this somewhere between
5 months x 4 weeks x 3 days = 60 times
and
5 months x 4 weeks x 4 days = 80 times
Now take off any time when you were on holiday / off sick / whatever. Then double the number of days when you did it as GTR may very well want you to pay for an anytime single fare each way.
But as
@AlterEgo says, don't send any of this in to the railway yet: at this point it's a matter of being prepared for what they might ask you.