The most i've used is 7 Keyboards at the same time, i mean around me and used all of them.
I used them few times like that but i usually never play with less then 4 Keyboards. Its mostly a
PA arranger, Prophecy always there and there is always a Motif or Triton/M3 with Casio CZ101.
And thats how most of us play our music.
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Cubase 8.5 Pro. Windows 7 X64. ASUS SaberTooth X99. Intel I7 5820K. ASUS GTX 960 Strix OC 2GB. 4x8 GB G.SKILL.
2 850 PRO 256GB SSDs. 1 850 EVO 1TB SSD. Acustica: Nebula Server 3 Ultimate, Murano, Magenta 3, Navy, Titanium.