How can I supply 17 showers without a drop in water pressure?
This is a calculation which depends on supply pressure, flow required, and distance. To avoid unacceptable pressure drop is simply a matter of sizing all piping components properly. It is easy to design a system which minimizes pressure drop and there are numerous on-line calculators you can use: link to calculator.

Your bigger problem will be hot water supply. At 2.5 gpm (a conservative estimate for water-saving type shower head) you will use 42.5 gpm (assuming all showers are in use), a good portion of that will be hot water (ratio unknown as is dependent upon delivery temperature and mix temperature).