According to the latest leaked slides, it appears that Intel's 9th generation Core Coffee Lake processor family will bring the first 6-core SKU to the mainstream desktop lineup.
While Intel previously had 6-core CPU, which was a part of the 2010 LGA1366 HEDT platform with Core i7 Gulftown CPU, there was no mainstream desktop SKU with more than four cores. It appears that this will change with the launch of the 9th generation Core Coffee Lake series, scheduled for the second half of 2018.
The Intel Coffee Lake architecture should launch after hte 8th generation Cannon Lake CPUs, which should probably only launch as notebook CPUs and will not come to the desktop market.
The 6-core Coffee Lake SKU is expected to be based on 14nm manufacturing process with a die-size of 149 mm2. The quad-core parts won't be made from the same silicon but rather based on a different and smaller 126 mm2 die.
Of course, we have yet to see Intel's 7th generation Kaby Lake desktop CPUs which should launch early next year so there is plenty of time for Coffee Lake rumors.