I am using Apache2 for hosted PHP apps, and had planned to follow @edbo’s production server NginX/Gunicorn recipe, but if you post your adventure I will eagerly follow to decide if I should instead my existing Apache stack. I expect an NginX/Gunicorn stack to be more efficient in terms of server CPU cycles and memory, but it will also require more learning and maintenance effort.
You may find the “How we used Tryton to build an insurance ERP” presentation at TUB 2016 of interest because it describes in considerable detail the challenges Coopengo surmounted to achieve their desired level of operations (the presentation refers to 1M invoices per month for 1M insurance contracts, and iiuc the video mentions the number of simultaneous users). The presentation is a few years old, but I think the technology is still relevant (spoiler, they use NginX).