From PCPartPicker Website. Here are all the compatible Motherboards for your Ryzen 9 5950X: https://pcpartpicker.com/products/motherboard/?compatible_with=Qk2bt6&__cf_chl_captcha_tk__=pmd_CuYk...
Click on the motherboard at the above link and it will tell you if it supporsts Dual GPU Cards or not.
Check the Motherboard you choose and go to it Manufacturer Support site and see if it supports Dual GPU cards. Most of the newest Motherboards support a minimum of 2 GPU cards.
Most sound cards use a PCIex1 lane so it won't be an issue.
As for the M.2 check the Manufacturer's Motherboard Specs and see what lanes it uses or disables when installed.
According to AMD the RX6700XT requires a minimum PSU of 650 Watts.
Go to this PSU Website and input all the hardware you will be using and it will show you the Minimum PSU Wattage required to run your PC: https://outervision.com/power-supply-calculator
NOTE: Personally with two high powered GPU cards and a high power CPU, I would consider a 1000 Watt Metal rated PSU or higher.