Previously, it was rumored you might need the Z390 for Intel's eight-core chips, but this rumor turns out to be false.
The support will likely come in a BIOS update, which should eventually roll out to all 300 series motherboards.
Intell allegedly has two 8-core CPUs coming with 9th Gen series: the Core i9-9900K and Core i7-9700K. It is quite possible that more octa-core parts will be unveiled in the future.