I think Hungarian only uses plural, when it's not redundant, so when no other parts of the sentence shows that there are more than one things. (I don't know if there are any cases when this is not true...)
Here are some examples where you use singular in Hungarian and plural in English:
A gyerekek könyvet olvasnak. = The children are reading books. (This is similar to the sentence above.)
Az eladó almát árul. = The shop assistant sells apples. (It's obvious that it's not only one apple.)
három kismalac = three little pigs (no plural after numbers)
sok autó = many cars
néhány autó = a few cars
Nadrágot hordok. = I am wearing pants.
Koszos a cipőm. = My shoes are dirty.
These "pair of..." things are singular in Hungarian: scissors = olló, glasses = szemüveg etc. Similarly:
Kék a szemem. = My eyes are blue.
I don't know what are the general rules, I just thought of some examples. I hope someone can explain it better.
So, am i correct that this sentence implies that there's a group of young men and every (or most) of them are wearing a shirt whereas if it would say "A fiatal férfiak ingekben vannak" that it would imply that there's a group of men and every (or most) of them are wearing multiple layers of shirts?
-n/-on/-en/-ön means "on" and -ban/-ben means "in".
Here are some examples:
A férfiak ingben vannak. = The men are in shirts. (The men are wearing shirts.)
A férfiakon ing van. = There are shirts on the men. (The men are wearing shirts.)
These two sentences mean the same.
A férfiak ingen vannak. = The men are on shirts. (The men are sitting/standing on shirts.)
A férfiakban ing van. = There are shirts in the men. (The men have eaten shirts.)