Quote from: din djarin on Today at 08:41:23 PM

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.

Be careful here, if your GPUs will consume 400watts, you need more than 500w, because you have CPU, motherboard, ram etc to consider.
Here you can calculate an estimate: https://www.coolermaster.com/power-supply-calculator/
But if you want to be quick, a 650w gold PSU is good enough to power your 400w GPUs and the rest

Quote from: din djarin on Today at 08:41:23 PM

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 '

I don't think this is the best idea, if you consider to have more GPUs, I prefer to buy a stronger PSU like 850w or 1000w because usually is cheaper, have more warranty and will be less cables to manage.

Quote from: din djarin on Today at 08:41:23 PM

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

The PSU on a rig is probably the most important part, don't be cheap here
Good PSUs like Corsair, EVGA or Seasonic will provide you a better quality to power your entire rig without risks to burn your hardware, and the most important, your safety.
Your rig will be on 24/7, you will leave the rig in your house for example, it's better to have a trusted hardware instead of cheap ones