There are many screen readers people use. Some use only one, some use several. Some screen readers only exist on specific devices. The selection of screen readers might also depend on the devices people own.
On desktops, most users use the Windows operating system. Both NVDA and JAWS have a lot of users. Mac with VoiceOver is much lower on desktop. So for the desktop, I would choose to test at least with one of the Windows screen readers. If you have time or the possibility, it's of course better to test with as many screen readers as possible.
On mobile VoiceOver is much more popular than Android with TalkBack. So if you have access to an iPhone or an iPad, I would test with primarily that one. But again, the more screen readers and devices you can cover in your tests, the better the result will be.
I recommend checking out WebAIM's Screen Reader User Survey for more details about the most popular screen readers, browsers, and devices.