South Korea is playing four friendlies before the World Cup. Their last Group Stage game will be their seventh game in 32 days. I don't know what the average is, but USA played three friendlies each before World Cups 2010 and 2014. I think four friendlies could make too many chances to get injuries.
I answered before reading your reply. I was also considering the March friendlies. With rosters of 23, you can play the equivalent of 90 minutes per week if you are rotating your squads, and still play 7 matches in 32 days.
Yeah, 4 seems like a lot. Aside from injury risks, you don't want players to get sick of the sight of each other. Which might happen if you play 4 friendlies and then another 7 WC matches. Remember that players are living and eating together throughout this time, its not like club football where they go back home to their families every day after training. I think the right amount depends a bit on the team and their goal. For real WC contenders I think the ideal number is two max. You figure such teams have enough to get past the group stage even if they aren't clicking on all cylinders by the time the first group match comes around. For teams like Morocco or Iran who have to be playing at their absolute best when they play their first WC match if they are going to have any chance of advancing past the group stage, maybe 3 or 4 friendlies makes sense.