There is only ONE kind of motherboard you can use with that CPU and that is a dual channel chipset. It doesn't matter if you use a Z series unlocked board, an H series locked board or a less expensive business class chipset. All of the motherboards compatible with that processor will be dual channel. Also, there is no i7-9900. Only 9900k.
So, let's not confuse having four DIMM slots (ATX form factor motherboard) with having only two DIMM slots (mini ITX and most micro ATX motherboards) with how many channels the motherboard supports. Support for X number of memory channels is dependent on the platform or chipset. Motherboards that support more than two channels are generally reserved for enterprise or server class platforms such as X or Xeon processor models.
For AMD, it would be Threadripper class processors that have high channel memory support.
It is not ignorant to ask such questions. It is ignorant to not know and make mistakes because you didn't bother to ask. Well, it's intentionally ignorant to do that anyhow. As I'm sure you've heard before, the only stupid question is the one that doesn't get asked. Better to ask, and know, then make assumptions and end up wrong.
Feel free to ask any questions here you are uncertain about, at any time, always.