Hi LavX,
Almost all cards, from lets say R7 370 and upwards can support 6 screens when using an MST Hub.
To make Eyefinity feature work, you will need to use the DP output on the card and when using Screens that have no DP input you will need to use active adapters for every screen after the third. An MST Hub is an active device so that would mean you can use “normal” cables to connect screens to the MST HUB.
Now if the GPU will be powerful enough to display your application(s) will depend on the application, the resolution/frequency of the screens as well as your own acceptance level.
I would not advice to connect 6 screens 1080P 60Hz to a R7 370 level card when you plan to run independent high resolution video’s on all screens. For that i would advice you to go at least for an R9 380X or even an R9 390(X) that has double the pixel rate of that of a R9 380X.