Four coin flips.
The Phillies-Brewers game has been rained out (update: Mets-Braves as well), but Citizen’s Bank Park will be home field in the event of an NL East tie with the Mets, or a wild card tie with Houston, Milwaukee or St. Louis.
The Brewers did manage to win the right to host the Cubs if there’s an NL Central tie.
MLB.com’s Tom Singer further explains:
The coin flips do not address two other possible scenarios: Three-way ties, and ties involving two teams from the same division already assured of postseason participation.
In the latter event, the first tie-breaker would be the head-to-head regular-season series between the teams to determine which club is the Division Champion and which club is the Wild Card….
Should three clubs finish the season with the same winning percentage, one as a division winner and the other as a Wild Card, playoff games would be played as follows:
¢: The two teams tied for the division lead play the one-game tie-breaker, with the winner being declared the division champion.
¢ The losing team in that game then plays the club from the other division for the Wild Card.