Everything below is quoted from Amazon's own help pages, with the page and the date it was read.
Source: "Make Your Book More Discoverable with Keywords", https://kdp.amazon.com/en_US/help/topic/G201298500, read 2026-10-11.
"Use up to seven keywords or short phrases."
The page does NOT state a per-box character limit, so listing_check.py does not measure one, and says so on every run.
Keywords to avoid, per the page: information already in the title or contributors, words already in your categories, subjective quality claims ("best novel ever"), time-sensitive statements ("new", "on sale", "available now"), information common to most items in the category ("book"), spelling errors, spacing or punctuation variants, anything misrepresentative, brands you do not own, quotation marks, Amazon program names ("Kindle Unlimited", "KDP Select") and HTML tags. The page says its list is not exhaustive.
