Not sure if this is the best place to post this, but I wondered about this recently. I took three seemingly-solo Youtubers as examples at different scales.

Penguinz0 has 16 mil subscribers, and as far as I know his uploads are just him talking into the camera, giving his opinion about recent things in games and pop culture (I could be very wrong about that, I don't know why he's popular). Looks like he uploads at least a few times a week, so that could be manageable.

Videogamedunkey has 7.5 mil subscribers, and he used to do weekly uploads until very recently, I think. Given that his humor and editing style is a big part of his content, I'd be surprised if he's got anyone else working on his vids.

Northernlion has just over 1 mil subscribers and uploads multiple videos daily, and (in)famously refuses to do any editing. He just basically streams on Twitch all day and uploads clips from that.

I would imagine the day-to-day running of the business, i.e. answering emails and phone calls, would be where having help is most advantageous as you grow, but it also probably depends a lot on what kind of content you make. How big can you get before it's too much for a solo operation?