I just want to come back and confirm that I have three working, active numbers on an iPhone 13 Pro Max.
One is a US Mobile eSIM that I will use for voice and text coverage where T-Mobile is weak. (US Mobile is Verizon.)
Another is a T-Mobile Magenta Max eSIM line that I will use as primary wherever I can, and with a QCI = 6 it is quite speedy even with one or two bars of LTE.
The third line is a free virtual line in Digits, which T-Mobile provides to Magenta Max users as long as you're also using the Scam Shield application. I have been using this number as a business contact for quite some time and it works fine for voice and SMS. It will not use iMessage, of course.
All three numbers active at one time.
The physical SIM slot is empty and available for another line. Any line installed via the physical SIM would work as long as I turned off one of the eSIM.