What is the maximum resolution for using two external monitors with a Macbook Pro 15" Retina?
It can support 2 external screens over Thunderbolt at 2560x1600 or one 2560x1600 on Thunderbolt &amp; one at 1920x1200 via HDMI (There is a linked report of getting all 3 working, but with overheating issues.)

Everymac is a good source of info for this sort of thing.
http://www.everymac.com/systems/apple/macbook_pro/specs/macbook-pro-core-i7-2.7-15-mid-2012-retina-display-specs.html