More than a sizing question, this seems to me like a brain teaser.
The key thing here is to see how many floors does the building have above the 12th floor (e.g., only count what goes from 13th floor up). Everything else does not make sense, and precisely for this I would not follow the approach of calculating # of people, verage trips, etc. It would be flawed.
Only for those floors from the 13th up, I would estimate:
- Total number of people
- Average times they go below the 12th floor, for which you need data like:
- Where the cafeteria and leisure spaces are located
- Where are the meeting rooms located
Hope it helps!