I'm pretty happy with the moderation here -- its one of the smaller non-beta SE communities and we have good coverage and good behavior (usually). The number of moderators needed depends on the size of the community (and I think the system does the determination of this on its own).
Once you have 20k rep, you have most of the tools moderators have. Most of the extra abilities moderators have are needed very rarely. Enough users have high enough rep to not need binding votes very frequently. We don't lock posts very often, need to suspend/delete users very often (*) and our tags are relatively clean, and I've never come close to my flag/close/delete/etc. limits.
Converting answers into comments (and better yet, the reverse ability if they allowed it) would be something I'd use occasionally, but I can flag these anyway so its no real loss. Its usually pretty easy to reach the moderators on chat here anyway.
However, if we are allotted 5 moderators and one hasn't logged in for nearly 5 years, I think it is time to replace that one. It may or may not be necessary for the moderation load, but as a just in case, it might be useful.
Finally, I'd like to note that I don't want to be a moderator -- I've got enough other things to do these days that I pretty much cast votes/flags, write comments and sometimes answers. I rarely have the time or will to go through the review queues unless someone points something out in chat.
(*) I can only think of two users of note who has been suspended, and I wouldn't have done either. Content outweighed attitude in my opinion, though I fully understand the (justifiable) decision to suspend them.