I can tell you that the builtin touchscreen of my Microsoft Surface Pro 6 works splendidly under Ubuntu 20.04-LTS. My girlfriend will soon test a touchscreen with Ubuntu 20.04-LTS for lab use, so that will provide another data-point within the next week or two i guess, a positive one i hope.
I know of at least one RaspberryPi 7" Touchsceen i own that doesn’t work well, because it violates the touch protocol specification, firmware bug i guess.
Unfortunately i was not ever able to test two touchscreens at once, due to lack of a 2nd touch screen apart from that broken RPi 7 screen, so i don’t know what the multiple touchscreen support is like, ie. if any bugs remain. I think all the reported problems on the forum were always only wrt. a setup with more than one touchscreen. Single-Touchscreen worked. And setup with multiple screens but only one of them a touchscreen also worked with some small configuration change.
In general, half-way modern USB connected touchscreens that use the Windows 8 multitouch protocol, and maybe have some kind of sticker a la “optimized for Windows 8” or whatever, should work well. Iow. all modern USB touch screens. Ofc. it is a should, because some of them have firmware bugs, so they don’t support the protocol properly and that’s where the fun begins. Cfe. this list in the kernel for “Windows 8 multitouch compliant” monitors that are mildly broken in one way or another and need special workarounds.
It is now possible to contract myself for looking into such things, although i’m pretty much booked out for the rest of this year and early next year, except maybe for smaller work without hard deadlines. https://www.psychtoolbox.net/
In such cases one would have to pay for work time and parts and materials, e.g., a touch screen or touch screens.