If you need to run only 6 displays, you can buy sapphire-gpro-6200
https://www.computeruniverse.net/products/90677337/sapphire-gpro-6200-brown-box.asp
It's basically newer, cheaper Radeon FirePro from Sapphire. I went with 2x gtx 1050 TI though, because I wanted semi passive cooled cards + 8 displays option. If you'll spend a lot of time glued to computer screens, you don't want to hear your PC running, it can drive you crazy, trust me
You could also go with gtx 1050 with 4 display outputs + GT 710 if price matters to you. It might be better option then going with Firepro or GPro 6200 as you are required to have active display adapters for more than 2 monitors. After the first 2 monitors all will require an Active adapter between the card to the monitor.
Cheapest of the cheap is to buy 2 GeForce GT 710 GPUs for 30 bucks each and enable Internal graphic processor (IGP) in your MB Bios. https://www.alternate.de/MSI/GeForce-GT-710-1GD3H-LP-Grafikkarte/html/product/1370152?
That will give you support for at least six monitors in extended mode, but not one big screen "eyefinity" mode, which as a trader you don't need.