Enough to say whatever it is you're saying. It will depend upon what you're writing about. I've read horrible books with thousands of paragraphs, and I've read great micro-fiction of no more than a couple of paragraphs.
Perhaps more useful are some guidelines about how many sentences to have in a paragraph. Again, these are not absolute limits but I was told once that there should be at least 3 sentences, and no more than about 8 in a paragraph. Again, not an absolute limit - if you've got short sentences you can have a few more. If they're all long sentences, you''ll probably end up with less of them than in a paragraph composed of short sentences.. Longer paragraphs are harder to read, so you don't want a paragraph that takes up half a page.
Each paragraph should ideally cover one main idea, or topic. When you start a new idea, or topic, or separate train of thought, start a new paragraph. Of course, if you're writing dialogue then it's a new paragraph each time you switch speakers. So, you'll need enough paragraphs to get across all you are trying to communicate.
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