How many watt meters are required in three phase power measurement?

It depends. If the system is a grounded or ungrounded Delta or ungrounded Wye connection (no neutral), then you need two. For an open Delta with a center-tapped ground, or Wye with a neutral, you need 3. Some monitoring systems also add a fourth, to measure ground current, although it is not needed to calculate power. Most measurement devices will accept up to 480V directly, for more than that you also have to add potential Transformers in a similar fashion to reduce the mains voltage down to that range or lower for the system to measure the voltages.