Hashtags have different purposes and their efficiency varies across every social network.
Viral accounts on Instagram (not the real world celebrities) often shoot up to 28-30 hashtags to bump up their exposure and grow their followers.
With Twitter, a couple of hashtags per tweet are optimal. Going over is spammy, and many posts don’t really make sense including hashtags anyway.
LinkedIn, 2-5 hashtags in longer posts are fine, especially tapped in at the end.
Hashtags could be used:
For events and specific communities
For group chats (multiple people sharing content per campaign)
For targeting a category or a designated keyword
For tracking custom content individually, i.e. designing your own hashtag for a certain type of content
Tapping into popular hashtags may help you. Then again, don’t forget to consider your target audience and whether the readership will fit into the right group of people you write for (or sell to).