“I can’t even explain how much of an asshole he is.”
How can a similar phrasing be constructed for plural assholes? I literally have no idea. Some possibilities that don’t seem right:
“I can’t even explain how much of an asshole they are.”
“I can’t even explain how many of the assholes they are.”
“I can’t even explain how much of an asshole they fill.”
“I can’t even explain how many assholes they have.”
Any ideas?
I can’t even explain how assholish they were.
I can’t even explain what a bunch of assholes they are.
I can’t even explain what assholes they were.
I can’t explain what kind of assholes they are.
I can’t even explain how much like assholes they are.
I can’t even explain how many of them are assholes.
“I can’t even explain how much of an asshole each one of them is.”
I can’t even explain all their assholes.
Nah, I think FloatyGimpy nailed it. 
Donkeys’ holes? :dubious:
“I can’t even explain how much of an asshole he is. And that goes double for the rest of them.”
mmm
This.
Although what is really needed is a collective noun for a bunch of assholes.
I’d like to suggest an odour of assholes.
I second the vote for:
“I can’t even explain what a bunch of assholes they are.”
although the words “a bunch” seem to add something that wasn’t in the original.
So how about just:
I can’t even explain what assholes they are.
Interesting that several of the suggestions have changed the original into the past tense. 
A pucker of assholes would get my vote.
Oh, and FloatyGimpy’s solution sounds the most natural to this Midwestern American.
This will depend upon what sort of plural we are talking about. It might be people as they function as part of some sort of organization or corporate entity. Then I think that something like “The IRS: I can’t even explain how much of an asshole they are,” would be appropriate. (Note that although you can refer to organizations in the singular, you cannot say “The IRS: I can’t even explain how much of an asshole it is.” An organization as such cannot be an asshole, but it can, potentially, turn its members or minions, when functioning as such, into assholes.)
On the other hand, you might be wanting to talk about some group of individuals who all just happen to be assholes regardless of the fact that they are members of the group. Then, for maximum clarity and grammatical correctness I think you would want “I can’t even explain how much of an asshole each and every one of them is.” That may seem to prissy and pedantic, however. A more demotic, slightly less “grammatically correct” alternative, that will sit better with the term “asshole”, might be "I can’t even explain how much of an asshole they all are."
The trouble with FloatyGimpy’s solution, and several of the others, is that it actually introduces new concepts (“bunch”) or omits them (“how much”), and thus risks subtly changing the meaning.
What this thread needs is Benicio del Toro to pronounce each version in turn so that we can more accurately judge.
I can’t even explain the extent to which all of them are assholes.
a flatus of assholes? a sounding of assholes? an explosion of assholes???
more suggestions for the collection…
How about:
My vocabulary is somewhat lacking when it comes to conveying this group’s verisimilitude to a gathering of rectal sphincters.
I can’t explain why their hovercraft is full of assholes.