No single number of keywords flips the switch. A title starts hurting click-through at the point where a person scanning the results stops reading a clear promise and starts seeing a pile of keywords. The damage is about how the title reads to a human, not how it parses for a crawler, so the threshold lives in perception, not in a density number.
One clear keyword phrase does a specific job: it tells the scanner exactly what they will get and gives them a single reason to click. Add a second, third, and fourth variant of that keyword and the title stops making one promise and starts making several half-promises, none of which lands cleanly. The eye reads it as trying too hard, and trust drops the instant a result looks built for an algorithm instead of a reader. That is the real failure mode. A stuffed title splits the one reason to click across too many competing phrases and converts none of them, even when it technically contains the exact words the searcher used.
The tipping point is the moment the title shifts from informative to desperate. A reader feels it before they can name it: the title reads less like “here is what this page answers” and more like “here are all the things I am hoping you searched for.” Once a result gives that impression, a cleaner competitor below it can take the click despite ranking lower, because its single clear promise reads as the more trustworthy answer.
So scan your own title the way a stranger would in a list of ten results. Find the point where it still reads like one clear human promise rather than a keyword dump, and trim back to it. One strong, specific phrase beats three crammed in that scare it away. This is the click cost of stuffing; whether Google also rewrites or devalues a stuffed title is a separate matter from the CTR damage here.