As for clocking, it typically only effects the upper end of the overclocking.  For a Z board you want to push to the limits, 2 DIMMs max, but if you are good to push it hard and need the extra RAM, then 4 DIMMs should be fine.
X299 (with a Skylake-X) is a different thing, as the memory controller is designed for 4 DIMMs for quad channel and can use up to 8.  There are certain brink OCs that you might need to hold off and go 2 DIMMs for, but honestly are you using this board or benching with it.  If this is a bench only system, you will use 2 for some things, and 4 for others.  If this is a daily use system, you play your games, check your email, clean up the videos you took of your kids sports event for the family, etc, I would say 4 would be best, as you will have substantially better memory bandwidth for anything that needs it.  As for gaming, it completely depends on the game, will it help Overwatch, no, will it help Doom, highly doubt it, will it help Galciv3, very likely, as it uses a ton of memory to maintain an entire galaxy, as well as having new optimization for high thread count processors.  I use that example as "gaming" is not a completely blanket statement.
FYI Kaby Lake-X is optimized for 2 Sticks as it is a Z series CPU on a Socket 2066.