The limit isn't the number that matters
Every platform has a hard limit — 2,200 characters for an Instagram caption, 280 for a post on X, 3,000 for LinkedIn — but the number that decides whether people read on is much smaller. Instagram shows roughly the first 125 characters before “…more”; LinkedIn shows around 210 before “…see more”. Whatever falls after that line is only seen by people who were already interested.
So write the first line as a hook: the benefit, the surprising fact, or the question — not “Hey everyone!”. Put hashtags, credits and links at the end, where they don't push the hook out of view.
How characters are counted
This counter counts what you see: an emoji, even one built from several code points (like a family or a flag), counts as one character. Most platforms do the same. X is the exception: it counts every link as 23 characters whatever its real length, and counts emoji and most Chinese, Japanese and Korean characters as two. The X bar applies those rules so the number matches what X will tell you.
Line breaks count as characters everywhere. On Instagram, several blank lines in a row are collapsed, so use a single line break between paragraphs.
Hashtags: how many to use
Instagram allows up to 30 hashtags on a post, but three to five specific, relevant tags usually do better than thirty generic ones. On LinkedIn and X, one to three is plenty; more reads as spam. YouTube uses the first three hashtags in a description above the title, and ignores all of them if you use more than 60.