Yes, "mountain" is a countable noun. You can have one mountain, two mountains, or many mountains. It can be used in both singular and plural forms, which makes it countable.
countable
countable
The noun 'hill' is a countable noun. The plural form is 'hills'.
cookies are countable unless you have brain problems
countable
Shark is a countable noun.
Duck as an animal is countable, but if you mean the meat it is uncountable.
Prawn - prawns is the plural - is a countable noun
countable
few is countable
Countable
The word hair can be both a countable noun and a non countable noun, depending on how it is used. For example in the sentence "Mary has long blonde hair", it is non countable. In the sentence "Dad is getting a few gray hairs" it is countable.