Power Supply
 
If anyone could help answer my questions, it would be greatly appreciated,

1) How do you calculate the number of watts needed for your GPUs?  In my case, I want to power two GPUs that require a total of 400 watts. So then do I buy a 500w power supply to cover my needs running at 80% efficiency.

2) If I build my rig with modularity in mind, would it be safe to assume that I need to purchase another 500w power supply to support another two 3060ti GPUs. So on and so forth with every two GPUs '

3) Should I only trust 80 Plus certified power supplies or can I cheap out for a Chinese model?