LinkedIn started its hashtags functionality in 2018, but extending out the reach of your niche is definitely useful in careful use of hashtags to secure a bigger number of followers. If you're running a business niche, you can follow hashtags of your interest/topics to see recent updates in your feed.
It's always better to engage your network on LinkedIn with professional or work related content, thereby typing the '#' and the phrase (eg #businesscontent) or the word directly. You can click on words with a hashtag, nowadays in blue, add these hashtag which are automatically prompted or recommended in the post before sharing.
The rule of thumb is maximum four (4). Why? It's because otherwise you look too spammy with too many hashtags (other social media platforms like Pinterest works best with 20 and you should follow what is the current algorithms' limits to get your post trending; For Instagram is up to 30 hashtags; For Facebook and Twitter maximum is 2.).
Basically, you can use relevant hashtags anywhere in your post,
And you can share an article, a video, a document.
Similarly, you can add a hashtag within your article written by you.
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