As many as you want. Our services are designed so that you can apply one or several to your feed.
You may also want to generate more than one FeedBurner feed from your original feed. For example, say you want both a mobile version of your feed, and another version of your feed that incorporates a style sheet. You simply create two FeedBurner feeds using your same original source feed. In the future, as we continue to add more services, you may want to create many FeedBurner processed versions of your original feed!