the help file says I can have a max of 8 GB RAM. Why can't I put in more? Or can I? It's a MacBook Pro 15" Mid 2012 model

Check the OWC site for available memory upgrades for your model/year computer. If your Mac is a non-retina display model it likely can accommodate 16GB. The Apple documentation was publiched at the time the computer was designed and built. 16GB RAM was not available at that time. Apple does not go back and update old publications for obsolete/vintage products.

everymac.com says:

Standard RAM:8 GB* Maximum RAM:16 GB**

Details:*As originally shipped, this model had 8 GB of RAM standard. Starting February 13, 2013, as a custom configuration of the MacBook Pro "Core i7" 2.3 15" Mid-2012 (MD103LL/A), it could have only 4 GB of RAM. RAM is installed as two matching modules, no slots are free.

**Apple officially supports a maximum of 8 GB of RAM, but third-parties have determined that this model actually is capable of using up to 16 GB of RAM with two 8 GB memory modules.

Check the full page (link at top) to make sure this is indeed your model.

It is best to only use memory from Crucial or OWC since Macs are very picky about the memory they use. Even when using memory from Crucial or OWC, make sure to only get the exact part numbers recommended for your Mac when using the tools on their respective websites to identify the correct memory.

Edit: I only saw the one reply when I posted my reply.

Relevant information: Upgrade RAM Macbook Pro (Mid 2012) 9.2 - Apple Community

I can confirm that 16gb is recognised...

I am running a mid 2012 MBP non retina with 16gb ram

MacBook Pro (15-inch, Mid 2012)

Processor 2.7 GHz Intel Core i7

memory 16 GB 1600 MHz DDR3