puwaha thank you so much for your answer. 1200W was not a realistic estimate, idk which card can suck up that much.

In where I live gpu prices are vary, like
rx 560s are about $169 (@amazon $120)
rx 570s are about $454 (@amazon $259)

For my current pc, I'm not sure that psu can handle that 2 gpu. I just need to understand how you calculate the necessary power, should I just sum up the stock wattages of gpus and cpu and put extra 100W when I'm looking for a psu right?

I had to rewrite the questions for shortening,

I have an Asus P5KPL-AM-SE motherboard which supports
1 x PCIe x16
1 x PCIe x1
1 x PCI

-I'm not on a good budget so, is it reasonable to start to build on it and grow with the profit?
-How can I calculate the right PSU power? I'm planning to build a 6 GPU rig at the end.
If I go with 6 GPU's that draws 1200W in total, how much should I add for the other components' power needs?

Thanks in advance