For 5, 6, 10 & 12 gallon batches, a typical recipe has us boiling approximately half the finished quantity, then adding (pre-boiled & cooled to sterilize) water to the desired finished qty/OG.
Ex: 5 gallon batch: boil 3 gallons of wort, end-up with ~2.5 gallons, add water to 5 gallons and/or...