Macbook Pro Early 2011 max ram support
The MacBook Pro from early 2011 will support 16GB of ram. The Macbook Pro 15" (non-retina) were sold with a maximum upgradeable ram of 8GB from the factory. However you can check OWC with your exact model number and confirm that it supports 16Gb.

OWC states the models that support 16GB here: http://eshop.macsales.com/shop/memory/Apple_MacBook_MacBook_Pro/Upgrade/DDR3_1333MHz_SDRAM

To get your exact model id follow those steps: (Lion instructions also apply for mountain lion)