Hashtag advice ages badly, and most of what circulates is from a version of Instagram that no longer exists. What follows is what each platform currently does with them.
They still work, but as classification rather than as a megaphone. Instagram uses them to decide what a post is about, which feeds recommendation. A handful of accurate tags helps that; thirty scattergun ones tell it nothing, because a post cannot be about thirty things.
The measurable part: hashtag-attributed reach is reported separately in insights. That is the only honest way to tell whether yours are doing anything — change the set deliberately between posts and watch that number, rather than trusting a list somebody sold you.
TikTok
Largely irrelevant to distribution. TikTok reads the video, the caption, and how people behave in the first seconds. Tags are a weak signal next to whether someone watched to the end.
Telegram
No algorithm to please at all. A channel post reaches its subscribers because they subscribed. Hashtags there are a filing system for your own archive, useful if your channel is large enough that people search within it.
Facebook and Threads
Facebook has never really used them. Threads treats a topic tag as a topic, and allows one per post — which tells you exactly how much weight it wants to give them.
The useful question is not how many hashtags to use. It is which platform is even reading them.
This is why the same caption pasted everywhere reads badly: a wall of tags that earns reach on Instagram is dead weight on TikTok and looks like a mistake in a Telegram channel.