For a new forum I think anything higher than 1 admin and 2 moderators is too much. I see all these new forums with multiple admins and more moderators on their forum than they have posts and it just looks so bad and unprofessional. People that just get into forums think that having more moderators will get their forum active and get more people joining but that isn't how it works
When we started the forum of our office website, I was the lone ranger there as admin and moderator. We added a moderator when the community exceeded 10 active members and got another moderator when the number reached 20 active members. For now, our office website's forum is not that active so just 1 moderator can easily do that job of monitoring. It is not easy to maintain a forum so part of the moderator's job is to post and create threads as well.
The amount of staff required is purely down to the discretion of the site owner and if your a member of a site or a forum, how many admin or staff members isn't really your concern.
Obviously the more active the forum, the more sections and threads it has, the more moderators a site will need and sometimes there are sites that seem to have more staff than members but again, if it's not detrimental to the site, it won't affect you as a member.
I think the owner, a moderator and an admin are quite enough for a new forum. If they create threads and keep the activity high level, the forum will be a success and even later on, the three of them, if they are dedicated, will be able to keep a forum protected from spam and keep the forum clean and active.
I'd just like to add that how active the admin are aswell can make a difference. A site might have a lot of moderators but not all of them might be doing their jobs every day on a regular basis. If this is the case I think a lot of site owners, instead of getting rid of the deadwood, will just get more mods, ones that will be active so sometimes seeing a site with a lot of mods and admin staff can be deceiving.
If the forum is just on the threshold, one admin along with a moderator are enough. Though, there are some aspects which need to be taken care of to maintain the quality of the posts, still too many staff at the beginning might spoil the cause.
However, if the forum becomes popular, and the number of users increases in the course of time, the count of staff members may require to be increased. The standard should not get compromised due to the lack of support staff.
Getting people you trust can also be an issue as well when it comes to finding staff to help run your forum.
Your not going to be paying them any wages, but you'll want them to feel as passionately as you do about your forum, and that can cause problems. If a member of a site is active everyday, provides well written content and as a lot of interaction then they will usually be good moderators, but they still won't put as much effort in to the site as you yourself will.