I can only speak for my own system. It may be very different than yours.
When batch sparging (which I do very occasionally), I heat 5g sparge water for a 5g batch. This is more than I need, but I use the excess for clean up.
When fly sparging, I heat 9g sparge water. Unless I have about 2g in the HLT, there isn't enough bead to spin the sparge arm. I also stop the sparge when the gravity reaches 1.010, at which time, there's probably another 2g in the lauter tun.
Whatever you do, make sure you have enough water. If you have too much, you can use the excess for clean up. If you don't have enough, you're in trouble.
After a brew or two, you can fine tune things to suite your equipment.