What is the total number of NICs allowed in a Mikrotik box?
We have a 2.7.xx box with 5 Realtek ethernet interfaces and while mikrotik can see and enable the 5th card, it will not use it. Can I get more interfaces using a new version? Or is the 4 port ethernet card Mikrotik has the solution?
At least 24 physical Ethernet ports are known to work. More should work also, but I have never seen a PC with more than 6 PCI slots. Note that PCI bus might be an issue running so many at the full speed - a standard 32bit/33MHz PCI has a theoretical (unreachable) maximum of 1066Mbit/s (from experience, about half of that in reality)
Regarding your problem, it might be something with that PCI port or with that particular card - try to swap cards…