Nate Nanzer, commissioner of Overwatch League, has made it clear that the league's goal is to expand to 28 teams.
In an interview with Inven Global, when Nanzer was asked how close the Overwatch League was to the goal of being like traditional sports, he responded, "I think we’re off to a good start. To actually realize our vision and goals, we are going to need more teams around the world. We will be starting conversations about season 2 expansion teams and we’re really focused on additional teams in Asia and Europe. Eventually our goal is to have around 28 teams around the world."
Nanzer has made comments about the Overwatch League's potential for expansion in the past, but this answer is the most direct so far. Nanzer also made the point to note in the interview that "the ultimate goal for OWL would be to become one of the premier sports leagues in the world."
The Overwatch League currently has 12 teams with many nations represented through the rosters of the teams.