2 would be my absolute limit. One is awesome. I can see when she gets old and unable to do agility anymore, needing to fill that void in my social life with another dog so I can keep training/competing. It would just be a bit of an overlap though, and then we'd be back to a one dog household.
I'm the same way, although, like Laurelin, I could see it maybe moving up to 3 for a brief period of time. I was actually quite happy with just Meg, until she started to let me know she wanted to cut back on agility. It is such a huge part of my life and social circle, I would have been lost. So, enter Gusto. Hopefully I have another 8 or more years before I need to worry about him wanting to be done.
I always want to have one dog actively training and/or competing. Even the last six months of no trialling has been depressing for me.