Wow SS. Awesome post! So enlightened. You seem to be on a positive streak! I'm so happy for you! *compersion glow*
There is one thing I would add from a poly perspective and that is for me I hold some guilt and can be apologetic for being poly sometimes. While you might be feeling fear about her possibly falling in love with someone else, I wonder if she is feeling scared about that too and feeling guilty and apologetic also for being a burden and for having different needs?
Our culture in north America has raised me to be mono, and I go against that. While it makes me whole and complete to have the priveledge of many lovers that love me in return, I will always feel on the outside because of how I have been raised. All I would suggest is to be aware of that and be the one that is the warm arms of acceptance and understanding. As much as you can be anyway. Your gf is not necessarily having an awesome time with all this as she loves you and still feels compelled to act on loving others also. Please keep that in mind.
As for how many? I find that two is plenty for any kind of quality and quantity time. After that it gets tricky and it really depends on the circumstance. I have time for a girlfriend who has an active life of her own with a husband and her kids and we both know our time is limited because of it. We are resigned to that and okay with it.
Just as I divide my time between friends and family I do with lovers also. Actually, I am better at negotiating time with people than most people I know. Especially mono people it seems sometimes. I see many friends and family where as many of my mono friends feel guilty about not getting in touch more often with people they care about. I have become an incredible organizer and time manager. I'm feeling proud of that today
You seem to be on a good track to making this all work. Keep breathing, talking, remember that this is the time in your life to explore things. Later in life it gets complicated and is not as socially acceptable. Not to mention there isn't as many people as willing to explore with you. Your time usnow to figure this out regardless. Enjoy the ride and make sure you keep holding on to what is healthy and for everyones best interest. Communicating and honesty are most definitely the ways to do that at the start.
Best of luck.