Hi Annam,
It's great that you're looking to expand your hours. My first observation is to make sure that the extension of the hours is properly warranted. And, you are correct. the first thing one would do is to check the historical information of people calling the centre after hours.
I do not know which ACD/call centre application you use. But for the majority of them, there my be a little report which keeps track of calls made after hours. If not, there are a couple of ways around this.
1/ Check the ACD reporting suite to see if there is a "calls offered" report and set the paramaters to look at a full 24 hour period rather than the hours you current work.
2/ Some PBX systems also have a call logger, to store basic call information for the rest of the business... you could ask your IT people to run a report of all the calls made to your inbound trunks broken down by hour/half hour intervals
3/ You could approach your carrier to provide a network inbound call report. This will give you the actual number of calls that your network provider sent to your phone system, usually broken down by hour... it can also tell you how many busy tones it sends out too... very insightful to ensure your number of inbound trunks are correct for the volume of calls.
Please let me know if I can be of any further assistance. I am keen to know if my comments have helped.
Kind regards
Gene