Too many--a reader's eye can skip over them as they're reading and they'll fade into the background.
TOO FEW and a reader is pulled out of the story with "Wait, what? Who said this??"
ALSO be cautious about breaking up your dialogue of a single speaker into multiple paragraphs without dialogue tags to indicate that the new paragraph is NOT someone else responding. Yes, you can have an unclosed quotation mark--but if the previous paragraph ends with internal narration or action, then the reader doesn't have anything to clue them in without a tag. You don't HAVE to start a new paragraph after an interjected internal thought/staging.
I totally overuse tags. I will "said" my way through every dang line of dialogue even when I have an action that will work for the beat/cadence pause I need instead. But the word "said" becomes invisible--excessive dialogue tagging is a risk you SHOULD take for clarity. (And something you can always pare back later in editing if it really is too much.)
One more dialogue tip:
It works best at the beginning and the end of a paragraph. Framing it inside a sandwich of action/thoughts/staging can result in the dialogue getting lost to the reader. Sometimes you need it, like if someone's frazzled or tripping but use sparingly.
These thoughts brought to you both by my current writing in which I am trying to rein myself in and let my action beats do a little more of the lifting, and also revisiting an old friend favorite read that lost me in the weeds of poorly framed dialogue this round.
A good, compulsive story with good characters/chemistry who you are rooting for desperately might overcome the technical issues in the actual writing as someone speeds through a book but--there's no reason to give readers an excuse like that to put it down, you know?
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