I am not too sure my method helps, it is my 2 cents.
In order to measure the capacity of Anvil hosted app, I create a timer in the client that sends add request to server every second, then count how many added row are made during 2 minutes.
Opening 10 or 20 tabs at the same time would definitely causes the app really slow. Adding a row takes more than 5 seconds comparing to opening only 2 tabs, which is about 1 secs.
I also test the same app with the open source server, the stronger my server, the more clients/tabs the app can handle. This configuration ( 24 CPU 96 GB RAM) can handle more than 100 tabs or more at the same time. The minimum one (1 CPU 512 MB RAM) can barely take care of 10 tabs.
This is just Vertical scale, if I can find a way to make the open source app server connect to a remote database, Horizontal scale scaling anvil app would be possible.