aleph_0, which is the cardinal of the set of integer. For uncountable, it is harder, depends of the axiom of choice, the "smallest" or "least dense" uncountable set is the set of all real number and its cardinal equal to beth_1, which cannot be proven. Still you can do the same thing to show the set of interest has a bijection mapping between the set of interest and R. You can use diagonal argument to show a set is not countable.