[GuidoK]

Complete flush takes less than 1L, but you have to know when the old fluid is drained/flushed so you don't flush extra fluid in vain.

I've weighed my 1L brake fluid tin can that I have left from my last flush (that is with a 6 piston front bbk, 2 piston rear bbk, which holds more fluid than the stock calipers) and the tin can still weighs about 400gr/0,9Lb (thats can+fluid). That's after a flush incl the rotation of the abs pump (you can do that for exemple with BMW DIS software)

So if you can determine the point where the brakes are bled/flushed just right (eg. by using a different color brake fluid) the 1L should be enough. If I estimate there is still about 200ml or so in the can.

Still it doesn't hurt to have an extra bottle standing by. If you buy your fluids locally most shops don't mind if you return an unopened bottle, especially when you ask them in advance.