how long is a piece of string?

... by which i mean, you give no details.
what kind of site is it?
how big is it?
how many users does it have.
how prolific are the users.
what are the managers/admins actually doing? overseeing posts, comments, checking reviews?

It's an intranet site with 88 pages and growing using Wordpress CMS. It has approximately 100 users accessing it multiple times daily. Managers do backend maintenance, manage editors, handle updates, create new pages, upload documents and manage nav menus.

Are you asking about how many people should be allowed access to do the same thing? That depends on how big your organization is and how well people talk to each other.

(A) "Oh, I didn't know that John had changed XYZ, so ABC no longer works."
(B) "I can't fix MNO because only Mary has access permission."

There should be only ONE master admin on any site. All the others are aub-admins, each specific chores, even if those chores are replicated by another in a shift/24 hour situation. In most cases that number of sub-admins (who manage moderators beneath them) is based on the traffic ... more importantly! ... whether the site is commercial or non-commercial.

Ordinarily that would be four people at top, one of which is above the other three. Those below come from workload or express experience/requirement of the site.

There should be only one with the keys to the kingdom, and is usually the owner of the site, or the owner's agent.

That's always been my practice and I've always set up websites that way, but I currently have a client/boss who is absolutely insisting on an additional four people (who have no web experience btw) having full admin access to the site.

I currently have a client/boss who is absolutely insisting on an additional four people (who have no web experience btw) having full admin access to the site.

in which case, what's the problem? i'd just do what they ask, as long as (if they are a client) they have written it in their specs.

As a site wonk for hire, you can only advise against certain practices that will lead to doom!

Make clear in that regard, and have them sign off on your advice which they intend to override so that you cannot be held responsible for the client, or any of their "admin" agents screwing the pooch!

In WP there are multiple permissions levels, they can do editing and see everything without being an Admin. Only Admins can boot other Admins or make account level changes. You can get complete information at the WP Codex section on Roles and Capabilities: [codex.wordpress.org...] if you need details to pass on to the boss.

Use one of the many user editor plug-ins and create a customized 'Role' (group) for those folks.

Only giving them enough juice to do what you feel they need. Get bold and name it 'Admin' if you like :-)

Then change their ID to have that Role assigned as their permissions and you're done!