5–15 tweets is the reliable range. Under 5 rarely justifies a thread; over 15 loses readers before the payoff. Aim for the shortest length that fully delivers your promise — then cut one more tweet.
Only in the final tweet. Links placed earlier suppress reach because the algorithm favours threads that keep people on-platform, and they pull readers away before they finish the value.
No hashtags inside the thread — they leak attention out to hashtag feeds mid-read. If you must use one, add it as a reply after the thread is published, not in the hook or body.