Depends on what is a defect. I’ve worked on some applications which are fairly simple and have a limited user experience. I have also worked on applications which use an adaptive design. They change how they look and how they use space based on the resolution of the display.
So if a test failed on a Samsung S7, it wasn’t the fact it was a Samsung S7. It was more than the resolution was 2560 x 1440. We might have a few devices which are 2560 x 1440 and they would all be considered equivalent.
I have seen things where the operating system plays a significant factor. An Android device versions an iPhone can be significantly different. So I’d want a mix of different devices and versions of OS to test again. When I first started testing mobile devices there was a HUGE number of variations. Version 1 to version 2 of an OS could have significant impact. Android vs. iPhone made a huge difference. We’d look at what devices were accessing the company and prioritize them as needed. In some cases it was just easier to write a different code base for each OS.
Today things seem to be stabilizing. It was just like when the Internet first started. The difference between browsers is very significant. At some point it just became unrealistic to support everything. Back then, you supported Internet Explorer but noted that 5.0, 5.5, 6.0, etc. were significantly different. Now IE Edge is using Chrome rendering engine. So which browser you use is far less significant.
Heck, even back in the 80s, which PC computers you supported was critical. The list of clone computers was significant. When 64 bit computers came out, manufacturers which used AMD processors was more important that Intel processors because more people owned computers with AMD processors. Intel and IBM was trying to kill the clone market. So all clone manufacturers had to make enough change to their system to not get sued by IBM. The difference where significant enough that I worked at companies which published a list of clones they would support. Now a PC is a PC.
I think, now, smart phones have stabilized enough that different hardware and OS are much less of a concern. Screen resolution still seems to be in flux.