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There's often a lot of confusion, but if you're looking for a general answer to the question, “How many sentences in a paragraph?” the answer is there are 3 to 8 sentences in a paragraph. The important key to take away from this answer is that it's a rule-of-thumb.
For example, you could have a one-topic paragraph describing Venus (with the next paragraph describing Mars) or a one-topic paragraph describing the colours of a sunset (with the next paragraph describing its reflection in the sea).