Buy 2 litres, try not to open the second one but be prepared. There's another variable...if you're going from no brakes to new pads, that's more fluid displacement than what's in the lines. Then I'd say you'd be safe with 1 litre. Also...depending on how fast you work, how you kink your lines, and generally how much fluid you drip out with changing the lines plays.
Get 2 litres and try to store the second one on the shelf. Lasts years on a shelf unopened, I'd only go 6 months once you open it. Leave the lid on as much as possible.