Yes, "suitcase" is a countable noun. You can have one suitcase, two suitcases, and so on. It can be used in both singular and plural forms, allowing for clear quantification.
The noun 'hill' is a countable noun. The plural form is 'hills'.
countable
countable
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
few is countable
countable
Countable
No, you cannot pack a suitcase within a suitcase.