I'm making a ceramic tile countertop for the kitchen. I'm using 3/4"
pressure treated plywood with 1/2" cement board applied overtop, with
a skim coat of thinset between the plywood and cement board. I'm
wondering about spacing of the cement board screws...I've read that 8"
spacing is recommended, but at the price of the cement board screws,
this seems like alot.

What about backsplash tiles...do I need to apply cement board first,
or can I apply the tile directly to the existing painted dry drywall.
If cement board is recommended, do I use the same spacing?

Why are special cement board screws required? Why can't you just use
drywall screws?