It's 25 items and 25 revisions each ;P
25? That does not really offer much in the way of creating say a large set of items. Seem lame.
You can delete them when you're done. The limit is only with the uploader; once your file has processed and you download it, you don't need to leave it in the uploader unless you want to. You can create as many items as you want for your own parks or put them in the Steam Workshop.