MacBook pro retina 15" mid 2015 - two monitors 4k@60Hz
From Everymac:


  *This model supports a simultaneous maximum resolution up to 3840x2160 on two external displays via Thunderbolt 2. Alternately, it can
  support a single display up to 3840x2160 via Thunderbolt 2 and a
  single 1080p display at up to 60 Hz, 3840x2160 at 30 Hz, or 4096x2160
  at 24 Hz via HDMI.