Five prominent Democrats are believed to be getting ready to run for Governor of California in 2018. See this story. They are Lieutenant Governor Gavin Newsom, Treasurer John Chiang, former Superintendent of Public Instruction Delaine Eastin (who is now a state legislator), billionaire environmentalist Tom Steyer, and Antonio Villaraigosa, former Mayor of Los Angeles.
The story also mentions that two Republican mayors of big cities may run for Governor. They are the mayors of San Diego (Kevin Faulconer) and Fresno (Ashley Swearengin). If all seven entered the race, it is not implausible that the two Republicans would place first and second, because the Democratic vote would be split up five ways.