"Kengät" also works for bread. "Haluan leivät".

This form is for when you are talking about a set of something. And shoes come in sets of two.

Hence "kengät" not "kenkää".

And if you prepare a bunch of sandwiches, you might say "haluan leivät" when talking about the set you prepared.

Like the other commenter said, the english equivalent of "haluan kenkää" is "I want shoe". "Haluan kengät" is used when meaning "I want a pair of shoes".

"Kenkää" is also used to mean "fired". As in getting kicked.