Yes, of course it is irritating. You use amount for one thing, and number for more than one. Hence, for example, the amount of mashed potato and the number of potatoes.
Back to apostrophes - it is potatoes not potato's
Those of us who had the rules of grammar hammered into our adolescent brains have to accept that if we had not been lucky enough to have been properly taught, we could well be getting it all wrong ourselves.
I love our language, I love the way it evolved from two distinct languages giving us such a wide vocabulary. I love the way we can stick to the more earthy, Germanic Anglo Saxon side of the language when we want to show we really mean what we say, or we can revert to the more airy fairy Norman French side, when we want to sound posh or superficial. Churchill stuck to the Anglo Saxon words in his 'We will fight them on the beaches' speech: the only Norman French word was 'surrender' as in 'we will never.'