The number of leaching chambers a property requires correlates directly to site conditions including the bedroom count (which represents gallons-per-day of water used), and the percolation or “perc” rate (which represents the rate of absorbency into the ground) where the septic system is to be located.  It also depends on whether an advanced treatment unit (ATU) will be used, or a conventional septic tank. 10 to 16 is a common range of required leaching chambers.  Septic system designers refer to NH Table 1016-1 to determine the leach field bed size, based on the number of bedrooms and perc rate.