After switching one of my Admirals to cruisers I tried a pure beam setup with 8 beams. In the first fight I wondered why I only see 4 beams firing. I first thought must be my autofire macro which spams the "fire all weapons" binding, so I increased the pause between the keystrokes. Got a bit better but still didn't saw all beams firing.
Then the surprise: Even without autofire macro and pressing the button manual with large delay between the strokes I still didn't saw all beams firing. Sometimes 5, sometimes 7, mostly 6 beams firing so I come to the conclusion the gamecode itself is the source of this issue.
It ended up mapping me all weapons to F1 to F8 and a modified autofire macro which spams F1 to F8 keys. First then I saw all beams firing.