How many hashtags, and which ones
Hashtags are a classification signal, not a distribution one. Once you know that, the right number per platform follows.
What a hashtag is for
A hashtag tells the platform what your post is about so it can decide which feeds to try it in. It is a label, not an audience. This is the single most useful thing to understand about them, because it explains every piece of advice that follows.
It also explains why stuffing does not work. Twenty tags across four unrelated topics do not enter you into four audiences; they make the label ambiguous, and an ambiguous label gets matched worse than a narrow one.
TikTok and Instagram: three to five, specific
Three to five targeted tags is the working range on both. Specific beats large every time: a tag with fifty thousand posts where your video can plausibly be among the better ones is worth more than one with fifty million where it is invisible on arrival.
On Instagram, pair the tags with a caption that says in words what the post is about. Instagram search indexes caption text, so the caption does part of the classifying job that people still hand entirely to hashtags.
Mixing one broad tag with three narrow ones is a reasonable default. The broad one gives the algorithm a category, the narrow ones give it a place to try you where you can compete.
X is the exception: one, two, or none
On X, hashtag-heavy posts read as promotional, and the algorithm treats them accordingly. Topic targeting there comes from the words in your post and from who engages in the first half hour, not from tags.
One tag when there is a genuine live conversation to join. Otherwise none, and spend the characters on the claim instead.
Picking them instead of reusing them
The most common mistake is a saved block of tags pasted onto every post. It is fast, and it guarantees the label is wrong most of the time, since it describes your account rather than this video.
Two things make picking cheap enough to do every time. The trend radar shows what is actually moving right now across TikTok, Instagram, X, Google and Reddit, including the themes appearing on more than one of them — which is a better signal than any single platform’s trending list. And every report comes back with recommended tags for the specific post you uploaded, at no credit cost, so the alternative to the saved block is not research, it is reading.
Reading about it only gets you so far. Score the post you were about to publish and see which of these it gets wrong.