"In this room all chairs are tall, not even any is short."
Translation:Ebben a szobában minden szék magas, semennyi sem alacsony.
Is thought "semennyi" would be used with uncountable nouns and "sehány" would be used with countable nouns?
Mennyi & its derivations can be used with anything (except when not because of set expressions.)
Hány & its derivations are used only when you could talk about specific amounts without adding unmentioned measure units. So
Három liter víz.
You want to ask "how much water?". You need to add liter in the answer, so the question is "Mennyi víz?".
You want to ask "how many litres of water?". You don't necessarily need to mention a new measure unit, so the question is "Mennyi/hány liter víz?"
When talking about numbers in maths.
Mennyi öt meg hat? Tizenegy.
You can think of it as countable/uncountable nouns, but tbh, it was a very alien concept for me when I first encountered it.