A hashtag turns a word into a link to every public post using it. People follow and search hashtags, so a few precise ones can bring a post to people interested in its topic. Broad, crowded hashtags rarely help; specific ones (a place, a niche, a campaign) work better.
Each network treats them differently. Instagram allows up to 30 hashtags per post, though a handful of relevant ones is usually enough; LinkedIn and X read best with one to three. Write them in a way people can read, using capitals between words, and keep a campaign hashtag the same everywhere.
