Why are there only 36 US governor seats up for election in November 2018?
Every state has its own constitution and election laws which specify the term of office and time of elections for state offices. There are 36 governor seats up for election in November, 2018 because there are 36 states which have a governor's race coinciding with the 2018 congressional elections.
As a single example, Indiana has no gubernatorial election this year because 2019-1853=166, and 166 is not divisible by four; instead it will hold its next gubernatorial election in 2020 for the term beginning in January of 2021.
Section 9. The official term of the Governor and Lieutenant Governor shall commence on the second Monday of January, in the year one thousand eight hundred and fifty-three; and on the same day every fourth year thereafter.
Wikipedia provides a convenient listing of the gubernatorial election schedule for all fifty states and five territories.