Generally speaking water to grain ratios will be between 1.25:1 and 1.5 to 1 (quarts per lb) for a mash with a sparge. If you're doing full volume BIAB, it goes up considerably. You can calculate your total water volume by determining your desired pre-boil volume (batch size plus boil-off) and adding between .25 and .4 quarts per pound for grain absorption (depending on whether you squeeze the bag). Then determine how you'll break it up into strike and sparge.
Example;
-5 gallon batch, 10 lbs grist, 1.5 gallons boil off
-6.5 gallons pre-boil plus 4 quarts absorption (.4 qts/lb grain absorption, no squeeze)
-7.5 gal = 30 qts/10lbs = 3.0 qts/lb for total liquor volume
For full volume mash, you'd need 8.3 gallons of space in your mashing/boiling vessel ( here's a handy "can I mash it? calculator:
https://www.rackers.org/calcs.shtml ). If you don't have that much space, you can do a dunk sparge so you could start at 1.5 qts/lb for strike and use the rest for your sparge. Like this:
-10 lbs @ 1.5 qts/lb = 15 qts (requires 4.55 gallons of space...5 gallon ice chest, bucket or pot)
-15 qts left for dunk sparging
You'll have to do a little trial and error to see where your numbers end up with your particular system and process. Take good notes and you'll be able to tweak your starting volumes and consistently hit your numbers.
Hope that helps.