The PPC Wiki includes a lot of very helpful information about beta readers which I would recommend you read. This one includes general information and this one is a list of all the people who typically are willing to beta on a regular basis, the fandoms they will beta for, and their emails. Keep in mind though that the list is four years old, so some of the information may be outdated. I hope this helped!

~ clairvoyant

Also, hi! Welcome! :)

Anyway, that's lovely, that your dad has been betaing for you and may continue. It's a bit unclear to me what you're actually asking here, though--are you trying to find more beta readers in advance? To see how many people might be interested in the future? Something else?

Betaing is rather dependent on people having time and energy and so on for it (as well as relevant canon knowledge/interest). LotR has always been a significant fandom here, though, so you're in luck on the latter front. As for time and energy, well, people have generally been managing to find a couple of betas even during the past year and a half, despite RL being more (and differently) draining and demanding. Especially for LotR, you should be able to find people. Just remember to have a mind toward patience and understanding, since there's a higher than usual chance that people will be busy or things will come up.

As an example, I'm pretty sure I've done almost no beta reading during this time, and most or all of it was either for something I was cowriting a follow-up to (upcoming :D) or something involving characters closely linked to some of mine (I think? I can't place the exact story, but I remember something like that). On the other hand, even before that I'd been busier than usual and was generally not betaing anything unless it was pretty short (~15 pages max), so I may not be the best/most usual example here. Ah well.

Anyway, welcome, enjoy yourself, hope some of this was helpful :)

~Z