I'm not sure on the quantity. Id put 1 litre of water, followed by a litre of coolant, water then coolant, then top up with coolant. Id say 4-5 litres should fill it. It will all mix when it gets pumped around.
It's 3 parts water to 2 parts coolant I think and the capacity is 7 litres, but if you have leaked "all" your coolant, you'll only get about 5 litres back in, then you need to top it up over the next few trips.