Thank you all for all the advice.

You are correct @philipma1957. I had a look at the box and there were 2 split cables which makes it in total 6 VGA connections. I honestly thought 6 PCIe means 6 sockets. Anyway, I can only connect max 4 GPUs as per all the recommendations which means I need to use a second PSU.

I'm going to limit the number of GPUs to maximum of 4 for one PSU. I will buy another PSU and connect the remaining GPUs to that one. I can connect the power supply tester (bridge connector) to power on the second PSU.