I am a single word.
one of me is spelled the same as many of me
but many of me is said differently than one of me.
I had the same idea as Dan Russell. My suggestion is:
Chassis. There is probably several other words of French origin that end in an 's' that aren't pronounced in the singular and that don't change in the plural.
EDIT: That was a lucky guess on my part! I didn't even notice it fitted the title.
The word you're looking for may be
The singular version is pronounced
While the plural version is pronounced