There are no specific rules to know whether a noun is countable or uncountable. There are some general ideas, however. If it is a liquid (or liquid-like) then generally it is uncountable. e.g. water, air, sand, hair, rice.
When you learn a new noun, make sure you check whether it is countable or uncountable.
Countable
For positive sentences with a single noun, we can use a/an. If there is more than 1 we can use 2, 3, 4 etc. or if we don’t know the exact number ‘some’.