One body, one lens. That doesn't mean that you can't have multiple cameras and multiple lenses. But when I go out to shoot, I pick one body and one lens that gets me into the right state of mind to shoot with that lens. When I go out with a 21mm lens, for example, I specifically look for things that will suit that lens and have that focal lens already in my mind when I look for interesting subjects and "see" everything in my mind in 21mm. I find that approach very liberating and leads to better pictures because there is no other gear to distract me and I can react much faster.
Example: two years ago I went to Argentina with an M4 and a 21mm lens and took great photos of landscapes, cityscapes, parks, etc. The following year I went to Argentina again, but this time with an M4 and a 50mm lens. With that setup, I looked specifically for interesting people and faces. So all day for almost two weeks, I focused on the fascinating people and wasn't distracted by looking for other subjects for which I would have to change lenses, filters, etc. And because I only had one camera on my wrist strap at all times, I was always ready to shoot within seconds notice. That approach works really well for me.