Hello and welcome to the forum.
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.