At the 2018 World Cup in Russia, 13 video assistant referees will be on hand for each match.
According to FIFA.com:
The selection criteria for these VARs were primarily based on their experience as video match officials in their respective association and confederation competitions, in addition to their successful participation in several preparatory seminars and FIFA competitions, where they enhanced their VAR knowledge and skills by using the system.
Each match at the 2018 FIFA World Cup Russia will have a VAR together with three assistants, AVAR1, AVAR2, AVAR3.
In addition to these 13 referees who will act solely as video assistant referees, some of the referees and assistant referees who have been selected for the World Cup will also act as video match officials during FIFA’s flagship competition. These appointments will be confirmed before each match.