The limit that matters is the fold, not 2,200
You can write a novella in an Instagram caption. Almost nobody expands it. The first 125 characters are the post. Put the hook, the product, or the question there; keep hashtags and credits after the fold. This tool shows that preview next to the hard 2,200 cap.
Bio and username modes exist because those fields fail silently: Instagram just stops accepting keystrokes. Counting before you paste saves a round trip in the app.
Hashtags without eating the caption
A row of 30 tags can be 400+ characters. Draft the story first, then add tags and watch both bars. If the caption bar goes red, delete tags, not the sentence people will actually read.
Frequently asked questions
Also useful: Twitter/X Counter · YouTube Description