I take as many cameras as I can bring. I'm sure there is a maximum number that I would bring, but I can never reach that number because I have to bring lenses and some clothing.
On a recent trip to Manhattan, I limited myself to three film formats: 135, 645, and a 6x9 rangefinder. The 135 and 645 cameras are SLRs, so I had several lenses for each. I also brought a DSLR which could share lenses with the 135 SLR. It was four cameras, 10 lenses, one tripod, and a sack of miscellaneous film, filters, and accessories. For years I had dreamed of making this trip. I enjoyed every minute of it. I made this trip expressly for photography and I traveled alone, so no one felt ignored or dragged-along while I traipsed all over by foot and subway with two camera bags, a tripod, and a Kodak Medalist II around my neck. When I do it again, I may leave the DSLR at home (no memorable shots with it). If you're vacationing with your significant other, only bring one camera.