Let's go over some technical aspects of DDR4 and why you're not going to find an 8 slot board for mainstream consumer CPUs.
The important thing to note is that mainstream CPUs, that is Ryzens from AMD and most Core processors from Intel (the ones not for some variant of LGA-2033) have two memory controllers. For example, this is a block diagram for a Zen 3 and 500 series chipset:
Also similarly with AMD's APUs, we only see two memory controllers:
Looking at the pinout for DDR4 288-pin DIMM slots (
https://www.micron.com/-/media/clie...-sheet/modules/parity_rdimm/asf9c512x72pz.pdf), we can note that there are a few pins labeled as "Chip select". These are used to address different RAM modules connected to the same memory controller. And if you continue looking through this, you'll notice there are four chip select lines (pins 84, 89, 93, 237). Thus there should be the possibility for four DIMMs per memory channel right?
Well if we look further down at the actual description of the pins:
Chip select: All commands are masked when CS_n is registered HIGH. CS_n provides external rank selection on systems with multiple ranks. CS_n is considered part of the command code (CS2_n and CS3_n are not used on UDIMMs).
UDIMMs are "unbuffered" DIMMs. i.e., the RAM modules that are in consumer computers. Which means that pins 93 and 237 are not connected in anyway, limiting the number of DIMMs per memory channel to two. Or in the case of consumer motherboards, a maximum of four DIMMs.
If you want 8 RAM slots, you're going to need to find an HEDT board.
EDIT: I noticed they talked about "ranks" and yes this is related to the single-rank/dual-rank setup in memory modules. And digging into this further, sites report that the only valid combination for a consumer board with dual rank memory, if you want dual channeling, is using two modules. You cannot have a dual rank, 4-module setup. Or at least, you can't do it with all of the memory accessible
(I wonder if this is a cause for why people ask why half their RAM is unusable?)