Well that kind of depends on your system and recipe and grind of the grain. What I do (FWIW), is first calculate my mash thickness that works best for my system - which is around 1.5 quarts per pound of grain. So if I have a grain bill of 10 lbs, I will use 15 quarts of strike water. Now here is where you have to do a couple of batches to learn what your system does. Once the mash is complete, you have to figure out how much wort you collect from your first runnings and how much you lose to grain absorpstion. Measure how much wort you have in the kettle, and use enough sparge water to get to your pre-boil volume. Keep your water temperatures in mind as well.
How do you know what your pre-boil volume should be? well that depends on where you live and how vigourous your boil is. I typically lose about 1.5-2 gallons or so to evaporation during the boil so I always aim for around 7.5 to 8 gallons for my pre-boil volume. This will also account for some losses in the gunk.
Now if you aren't sure of your losses yet, you can use some guestimates:
-mashing with 1.5 quarts per lb could yield around 3 gallons of first runnings with 10lbs of grain or so
with this in mind perhaps 4-5 gallons of sparge water could bring you up to your pre-boil volume
now these are just some rough numbers that approximate my system so they may or may not work for you. Others here are waaaaaaaaaayyyyyyyyyyy more experienced than me. This is just what I have done over the last year of home brewing so take it for what it's worth - a guestimate from another novice home brewer!
In any case have fun and just try different things and find out what works for you. There is a ton of great info in these forums and a lot of very knowledgeable people that are quick to help out.