Hitched says: "We don’t mean if their battery runs out (that’s just a bad, ill-prepared photographer), but sometime accidents happen: cameras break, lenses smash, external hard drives get damaged. Ask what they have to hand on those situations – a professional photographer will often carry three cameras with them."
Well, yes and no. A battery running out happens to the best of us but of course we'll have spares and chargers just in case. The yes part is that a professional photographer will have an extra camera and extra lenses, and extra hard drives or backup of images, however the no part is that not all photographers will carry 3 cameras.
When I first started I had one camera with two lenses, and as I progressed I was able to buy more. Equipment for photographers isn't cheap, I recently bought an older model Nikon second hand and it was still £850 just for the body of the camera with no lens, so most photographers starting out probably wont have 3 cameras. Batteries for the camera are £85 each too!
Personally I have 3 cameras, about 7 batteries, 4 or 5 lenses and chargers for everything. When I buy equipment now I try to get at least 2 of everything, and the insurance to match!