Facebook added hashtag support in 2013 and its users largely declined to adopt it. That history matters, because it means tags here carry a faint whiff of "posted by a scheduling tool" that they do not carry elsewhere.
Two or three, maximum
A Facebook post with ten tags looks syndicated, and syndicated content performs poorly in a feed built around personal connection. Two or three tags that a human would plausibly type is the ceiling.
Where they do work
Local business. Geographic and community tags still function, particularly in Groups and on local business pages. #shoplocal paired with a specific town name reaches people who genuinely browse that way.
Groups. Inside an active Group, tags help members filter a busy feed. Some Groups formalise this with agreed tags for post types — check the pinned rules.
Events. A campaign or event tag gives attendees a shared thread and gives you an archive.
Reels are the exception
Facebook Reels sit on the same recommendation infrastructure as Instagram Reels and behave accordingly. Tagging Reels the way you would tag Instagram Reels — three to five specific, descriptive tags — is reasonable, and the audience there is used to it.
What drives Facebook reach instead
Comment threads, shares into other feeds, and Group distribution. A post that starts a conversation in the comments will outperform a perfectly tagged post that does not, by a wide margin. Native video and photo posts still outperform link posts substantially.
Pages versus profiles versus Groups
Organic Page reach has been low for a decade and is not recovering. If Facebook matters to your work, Groups are where the actual distribution is. Tags inside Groups are a navigation aid for members rather than a discovery mechanism — useful, but a different job entirely.