I would proceed in the following way:
- Estimation of the number of passengers
- Estimation on the number of toilets needed (you have to consider the different areas of the airport and the amount of time spent in each one).
- Reduce this number to consider just normal WC and not urinals that you can find in men restrooms
- You can assume that the latter number is equal to the number of toilet papers that you are actually using
- At this point you should ask if you have also to consider toilet papers that are in the storerooms to replace the ones that you are using.
Hope it helps,