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I have a dell optiplex 390 over here that I want to run two gtx1660 super cards on but won't boot if I install more than one card on the motherboard, I'm using the pcie-16 slot and pice x1 slot with a unpowered riser, what could be causing this ?

You need to power the riser.

GPUs can draw up to 75w from the spcie slot.  The riser removes this load from the mobo and provides it by the riser but you gotta plug power to the riser.

I wonder if you are using a ribbon cable full slot extender instead of a usb cable type riser.  If using a full slot extender then the gpu will try to pull power from mobo...which it may not be able to provide.  Dell is not known for over building its systems.

Grab yourself some powered risers on ebay, they cost less around 5 to 7$ per one, the motherboard may not be able to provide all needed watts to the card so it won't boot up that is if you are sure that the second card is also running fine alone

It looks like the motherboard isn't getting enough power juice through the riser, you need to get a powered riser and also good power supply, if you still get same result you will need to go into BIOS settings and change from gen1 to gen2 or change to UEFI bootable from legacy bios, I've heard some was able to fix this type of issue this way

https://aliexpress.ru/i/32806274942.html
Buy such a raiser and a 600 watt power supply.
I did not find in the specifications which power supply you are using now, but a power supply less than 400 watts will not be enough for your farm.
As a rule, BIOS setup is not needed.

PCIe x1 slots are usually limited to 25 Watts, GPUs require 75W from the slot, always use powered risers.

You are right about the riser but not the watts, a single gtx 1660 super mines Ethereum algorithm at 80 to 85 watts, two of that gpu will do just fine on a 450watts power supply, that's not even up to 200 watts for the graphic cards only

Yup...usually with dells if you look at the mobo between the slots it will have printed what watt limit is for that slot. 

I'm using 750 watt power supply and I believe this should be enough for up to 4 different gtx 1660 super or even 5 because each gtx1660 super are using only 82 watts per card and they are same brand with Micron memory

What is this watt on MOBO looks like? All I can see here on the Dell motherboard is 1.5A/6V, I'm not sure if that is what you are talking about, anyways I've ordered for powered risers but it's going to take three weeks to get delivered to my door step, such a long time, now I have no choice but to keep mining with one card 😔

First pic down the page shows it.

https://www.dell.com/community/Desktops-General-Read-Only/35W-Limit-on-PCIe-x16-slot-Optiplex-790-SFF/td-p/4709629


It is screen printed right on the PCB.  Usually white print.

Dell optiplex 390 is different from 790 Small form factors, I guess graphic cards with no 6pin plugs works well in SFF Desktop PC, but it's better to use powered risers like many suggested too, pcie x16 watt is limited but OP said only two GPUs have problem running on the motherboard, I'm guessing the other ports aren't pcie x16

Dell optiplex is optiplex don't you think? the guy has a point there, those old desktops can't handle two GPUs in a single motherboard on their own, OP have to order for powered risers and the system wi be able to boot, maybe in future we will see new PC that can draw more watts from the pcie slots who knows?

Was going to say a gpu without 6 or 8pin power plug...is not much gpu....

In your city, raisers are not sold in online stores or online markets of used goods?
All beginners are recommended to use time-tested motherboards, I wrote about inexpensive configurations here
https://bitcointalk.org/index.php?topic=5313613.msg56250132#msg56250132
If you want to mine, then do your farm right.
I also started mining and farms of their unnecessary components, but then everything had to be redone