Two words u and v are said to be k-abelian equivalent if, for each word x of length at most k, the number of occurrences of x as a factor of u is the same as for v. We study some combinatorial properties of k-abelian equivalence classes. Our starting point is a characterization of k-abelian equivalence by rewriting, so-called k-switching. Using this characterization we show that, over any fixed alphabet, the language of lexicographically least representatives of k-abelian equivalence classes is a regular language. From this we infer that the sequence of the numbers of equivalence classes is -rational. Furthermore, we show that the above sequence is asymptotically equal to a certain polynomial depending on k and the alphabet size.