How can I display the occupancy rate or the number of reservations per day in the timeline or a specific row ?

1. If you want to display row summaries, you can add one more row header column and display the data there:

http://doc.daypilot.org/scheduler/row-header-columns/

2. If you want to display column summaries, you can add one more time header row (with groupBy = "Cell") and use BeforeTimeHeaderRender to show custom HTML:

http://doc.daypilot.org/scheduler/time-header-rows/

3. Another option for column summaries is to add one more resource/row at the end display the data in the grid cells using BeforeCellRender:

http://doc.daypilot.org/scheduler/cell-customization/

However, in this case the data will be scrolling with the grid (it won't be fixed like in option #2).