It should only take about 1 litre, but get 2.
I used about 1.5l when I last did mine.
Get a syringe and pull out most of the old fluid from the master cylinder before you start adding new stuff, but obviously do go too low.
If you are using one of those one man auto bleeding kits that use the spare wheel to push fluid through, make sure you don't exceed 1bar of pressure. So deflate the tyre to about 14psi first. Using a greater presssure causes a valve in the EDL sysem to close and you won't flush that part out.