I use 3 on some occasional gigs but mostly 2...on the 3 Key's setup I use a

Roland FP4 Piano, Yamaha S900 and and an Evolution MC-461c controler running Native Insrtuments B4.

Used to use 4 keyboards years ago and I mean 'years ago'...you can tell the age by what I was using....anybody guess what period in time it was ?

Fender Rhodes 73 Mk2
Clavinet D6
Korg 700s Synth
Logan String Melody

Rgds

Noel