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.
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
No, you cannot pack a suitcase within a suitcase.