As many or as few as you need to get your point across. I've sometimes looked at a paragraph and thought, "That's too big, I'll break it up," but I've never counted the sentences. One long sentence could occupy as much space on the page as five short sentences.
By the way, don't keep editing that one chapter. That's a recipe for never finishing. Finish a draft of the whole book, then start editing. Anything in a story isn't good or bad in itself, only in the context of the story as a whole. Until you have the whole story, or something at least vaguely like it, you don't know what context anything exists in. You might decide to cut the first chapter, because you started the story in the wrong place. If you'd polished every sentence until it shone, you'd be reluctant to cut it, even though you'd make the story better, because that would represent a lot of wasted time.
EDIT: In a fit of boredom, I wrote a quick and dirty program to count the sentences and paragraphs in my books, then divided one by the other to get an average. It comes out at between 3.5 and 4 sentences per paragraph. But don't feel that you have to make them all the same length. To my way of thinking, that would make for a boring read, more so than having some long ones and some short ones.