DDR5 motherboards should come with only 2 dimm slots as standard.

 It might be a bit early for this but so far with overclocking DDR5 the main thing I've noticed is that there are far larger differences in what memory speeds can be achieved based on a motherboard's memory topology than with DDR4.

1 DPC boards like the Unify X and Tachyon easily do 6800Mbps or more.

8 layer daisy chain boards like the Master, Carbon and Aero D have hard time with more than 6400Mbps

6 layer daisy chain boards like the Aorus Pro struggle with more than 6000Mbps

Various manufacturer QVLs mostly match my expirience. 

MSI Z690 Unify X (8 layer 1DPC) :

MSI Z690 Ace (8 layer daisy chain) :

MSI Z690 Tomahawk (6 layer daisy chain) :

Gigabyte Z690 Aorus Tachyon (10 layer 1DPC):

Gigabyte Z690 Aorus Master (8 layer daisy chain):

 Gigabyte Z690 Aorus Pro (6 layer daisy chain):

Asrock Z690 Taichi (8 layer daisy chain):

 

Asrock Z690 Phantom Gaming 4/D5 (6 layer daisy chain):

The only reason for a motherboard to have 2 dimm slots on a memory channel is to allow quad rank memory configurations. Quad rank memory configurations only make sense if you really need a lot of RAM. Since DDR5 has much higher densities than DDR4 the benefits of having extra dimm slots are smaller than with DDR4. Currently the smallest x8 IC based DDR5 stick you can buy are 16GB. In dual channel that's already 32GB of RAM which is a lot of RAM for most tasks. If that's not enough 32GB dual rank dimms already exist. In the future 24GB, 48GB and 64GB dimms will be available. Obviously for workstation motherboards support for more memory capacity at the cost of memory speed makes perfect sense. However for less memory intensive users having a bunch of completely empty dimm slots that hurt memory overclocking doesn't make sense. Especially when the extra dimm slots have such a large negative impact on memory clocks. I also suspect that a 1DPC 6 layer motherboard would clock better than an 8 layer daisy chain board.