You'll absorb about 0.1 gallons per pound, so you'll lose a gallon or so to absorption, then whatever your equipment loss is, and then you need to collect more preboil volume to account for boiloff (1-2 gallons per hour, depending on your burner/kettle) and then you also need to account for trub loss (cold break, mostly) that will precipitate out of solution when you chill your wort.
I BIAB, so I can squeeze my grains and end up with more like 0.05g lost to absorption and no equipment loss, so I typically use enough hot liquor to add up to ~8 gallons, so I will always have 7 gallons into my kettle no matter what, and have a very narrow pot so I boil off only a gallon per hour so I end up with 6 gallons of sweet wort out of the kettle. I lose a lot to cold break, betweek 0.5 and 1 gallons, so I always end up with 5 gallons into my keg.