It takes as many shifts of harm to take someone out as it takes. The amount is arbitrary, since the character may have more or less stress boxes, consequences, stunts, etc. Asking how many is kinda like asking how long a piece of string is.
To calculate someone's plot armor against getting taken out by a single attack, you'll need to add up the following, assuming they're willing to spend everything they can on avoiding being taken out:
- The size of their biggest applicable stress box. A character has 2 stress boxes by default, and may have 3 or 4 if they have any/enough physique or will (see adding tracks and slots).
- Their consequences (if they have them): minor (2), medium (4), severe (6), and their extreme consequence (8) for a minimum of 20 shifts, plus any other extra consequences added by their stunts/extras.
- +2 for each aspect they can spend a Fate Point to invoke. (You can only invoke an aspect once via fate points.)
- +2 for each free invocation they have on a relevant aspect. (You can stack an arbitrary quantity of free invocations, even from the same aspect. See that section, 4th paragraph, particularly the last sentence.)
- +2 for each boost.
- Add in the effects of any stunts or armor.
- +1 shift to be the one to finally take them out.
Assuming all but the first two points add to nil, this results in a minimum of 23-25 shifts (20 from consequences + 2-4 from a stress box + 1) to take out a player character in one blow, if they have no extra consequences. This excludes their defence roll, which will add ±4 and their defence skill, too.
If you don't quite manage to take them out (e.g. you only get 20), the character will still be pretty hobbled after this, and significantly changed by the experience of whatever happened to them in this attack, thanks to their extreme consequence!
In a practical situation, this is a lot of variability, so if you ask me how many shifts a character can take, I'll be asking you to show me the character and circumstances.
Bear in mind, though, this doesn't make the system deadly. The majority of the section on getting taken out is on character death, and basically says it's boring. Even if someone's got a shotgun aimed at them, they probably won't be taken out by actually being hit by it.
Further, you mentioned knocking someone out via consequences - that's probably the least reliable way to knock them out. You're probably better off using create an advantage to place a knocked out aspect on them.