So it can't be "boynunda birçok yaralar var" ??
did ''boyun'' mutate to ''boynu'' is it a loanword ??
Yes, boyun turns into boyn- if you add an ending that starts with a vowel.
It's a pure Turkish word. Other Turkish words do this, too, e.g. ağız "mouth", burun "nose", alın "forehead".
So it's not only loanwords like "fikir, fikr-" that do this.
I am interested whether there are any alternative ways to say the same thing, for instance 'Onun boynunda pek çok yara var'?