I use P3D with PMDG737NGX on one computer. My videocard had 5 ports. Works great with 4 monitors bur when I connect 5, one monitor turns off. What could be the problem? Maybe the computer has to low capacity...?

Can I use two videocards on one computer. I need to run 6 monitors....Or is it a possibility to use two computers....? What is the best?

Magnus

You don't give any system info, but 4 active outputs is the limit on a number of graphics cards.

There is no problem running 2 graphics cards, given a big enough case and adequate cooling, so long as your cpu can keep up the pace, and you may also need a bigger power supply.

With 1080p screens, I have 2 outside views on a GTX 1080ti and 3 instrument monitors on a GTX 1070. I find that is about the limit for acceptable fps on an intel 9900k running at 5.0 Ghz.

You can run 2 or more networked computers, but it's not entirely straightforward and the gains can be less than expected.

12 hours ago, MLU said:

I use P3D with PMDG737NGX on one computer. My videocard had 5 ports. Works great with 4 monitors bur when I connect 5, one monitor turns off. What could be the problem? Maybe the computer has to low capacity...?

There are, as far as I know, no graphics cards that will drive 5 displays at the same time, irrespective of how many outputs they have. 4 screens is the maximum for both NVidia and AMD. I have the same issue and I run a second GPU in that machine for screens 5-7. The only alternative is to use something like a TripleHead2Go to create a virtual display out of multiple screens, but that only works if you have multiple screens of the same resolution side-by-side, ie you want to create a super-wide display. 

If all of these displays are for outside views, multi-channel is the way to go for performance reasons, but as Biggles2010 said above, that adds a huge layer of complexity. But if they are mostly for instruments then for flat 2D gauges, 2 GPUs works fine and the performance hit is not terrible. Multiple 3D views of the VC will have a much higher performance hit. 

Maybe there is also a onboard videocard on your system. I have two in my Nvidiacard and one in my onboard Intel HD graphics, it saves a few frames on the workload sometimes.