The answer depends on how much grain is in the mash. I would recommend boiling at least 3 gallons and adding water slowly until you reach 168. Even better, using brewing software does the math for you. There are free software options and beersmith, which I use, is about $20. Beersmith has a free trial version as well. Also, doing a mash out is also not required, although many brewers prefer to mash out.
And here. I use Brewer's Friend for all of the calculations I do for brewing. Well except priming sugar, for some reason I've always used the Northern Brewer calculator for that. And the "Can I Mash It" calculator on Racker's that's on AnOldUR's link above is awesome.