If I may offer my 2p worth, this whole sentence seems extremely convoluted. Firstly, the first sentence in your text
The skulls of every human being have 22 bones
is incorrect. It implies that if you total up all the bones in all the skulls of every human being, you will have 22 bones. It should read
The skull of every human being has 22 bones
Secondly, the word
make do not become plural just because they end with an 's'. Consider the phrase
He serves as a waiter
He is singular, so the correct word is
serves, implying plural.
If there were more than one waiter, it would be
They serve as waiters
In this case,
serve indicates plural. So,, your grammar book seems to me to be incorrect; the fourteen bones are plural, but the phrase should be
Fourteen of the bones serve as the face and jaw.
To my mind, however, a better way to write both sentences would be the following;
The skull of every human being has 22 bones. Fourteen of these serve as the face and jaw.