K&F Concept M42 to Sony E lens mount adapter you are asking, Sony A6400 camera mount is E, so this fits you needs, please trust to purchase.
Any lens that use the M42 size screw in mount. The only other screw mount is M39 and these won’t fit, they would need a different adaptor. They will invariably be vintage lenses and you will only be able to use them in the aperture priority or manual modes, stopping town the iris when metering. Search on EBay for “M42 lens”, that will give you an idea of what you can find.