Words, characters, sentences, paragraphs and reading time, all updated as you type.
The everyday text work of writing day: how many words, what changed between two drafts, what the markdown looks like before you paste it into the CMS. None of it leaves the page until you copy it out, which matters when the draft is not yet ready for anyone else to see.
Read the guidelines you are writing against, because both conventions are common - some counters take everything between the first word and the last, others exclude titles, captions and notes. This tool counts the text you paste, so paste exactly the part the editor will count.
Short enough that the recipient can read it on one screen and still find the hook on the first pass. Outlets publish their own preferences, and a cover letter for a submission is not the same document as a cold pitch - check the guidelines rather than a general rule of thumb.
Counters disagree, and so do publishers: hyphenated compounds are usually counted as one word, an em dash without spaces may split a sentence into parts your editor counts separately, and numbers behave differently again. If a limit is hard, leave a margin rather than landing exactly on it.