is it better to use 2 or 4 ram slots

In this case, my suggestions will be that you should use multi-channel if your motherboard offers 4 RAM slots or more and single-channel if your motherboard only has 2 slots, then you will be able to upgrade it later on.

so a mobo maker will use 16 for the 16x pcie slot. they'll use 4 for ram. and 4 for all your sata ports, m.2 slots,usb connectivity and ethernet.

I use DIMM1-3 and 4 slots booted but not seen slot 1 RAM. How to fix these two problem? ... i tried put 2 sticks into slot 1 and 3, 1 and 2, 1 and 4, 3 and 2, 3 and 4.