Hi! The reason for the specificity of the requirements is basically just to filter out people who are into vTubers and are into the vtuber niche as opposed to people who just watch vTuber clips. Admittedly, the requirements were purposedfully made to be an easy enough gap to clear for people who are in the vTuber niche, as there hadn't been any previous studies that clearly stated their filter requirements (and how long do vTuber audiences usually watch/interact with vTuber content). This was done because the study wishes to observe how parasocial relationships can possibly affect one's well-being when observed in the context of vTubers as opposed to celebrities or other media personalities (due to the differences in peoples' perception of vTubers as a media personality).
It doesn't need to be with one vTuber specifically, just as long as you've interacted with vTubers in some capacity. And yes, you can definitely be parasocial towards multiple vTubers; think of it like having friends, you're not limited to only having one friend. The only caveat is that the relationship is usually a one-sided one (I say usually, as with some indie vTubers, they may actively hang out with their viewers, which brings on a different kind of limitation).
From the literature review I've done, parasocial relationships can be a good thing or a bad thing; it all depends on context, and how you're going about it. I know, that's not a satisfying answer, but I'm also withholding my hypothesis (as I don't wish to potentially affect the data/your answers by outright saying whether it's a good thing or a bad thing). Though, you're welcome to email me (listed on the consent form) if you wish to see the results after my thesis is completed. Hopefully that answers some of your questions!