I moderated a few decently popular subreddits for a while (a couple hundred posts a day, nothing crazy).
I will say that most moderation work on Reddit can be done independently of a particular sub and scales okay across subreddits. Like the additional effort of moderating 100 subreddits vs 1 would be nowhere near 100x. It would probably be closer to like 5x the amount of work.
And there are benefits to moderating many subreddits.
Most moderation is keeping your automod filters up to date to stop the never-ending onslaught of porn bots, crypto scammers, and obvious fake accounts looking to farm karma. Everyday I'd scan through both the stuff the filters caught and the stuff they didn't and tweak the automod settings to be better, then most of the time I'd copy paste those settings across subs.
The other hard part of being a mod is communicating with the other mods. Like I would only be online moderating one hour out of the day and so all communication with async with anonymous strangers. This again benefits from having mods span many subreddits because you can quickly get to know the other moderators in similar spaces and you all pass information around to each other.
For subs that require user verification this again is a better user experience if you moderate several similar subs because users can verify with you once and be approved across many subs.
The communities actually tend to do a pretty good job of moderating themselves via reporting and downvoting so even if there's only one mod on a subreddit they act way more like an appeals process than an initial judge of fate, most content moderation is actually outsourced to the community as a whole with the mods only responsible for curating a space free of obvious spam and acting as an appeals process for people who have content on the borderline.