Regarding wall anchor specifications
Yes, per anchor unless otherwise specified (and sometimes it is, e.g. in pairs). 

Accurate? This is advertising we're talking about, and assumes perfect conditions. I'd halve the number given.

Regarding your car example, obviously at some point the load will overcome either the structural integrity of the entire drywall sheet (as opposed to the local structure immediately around the anchor) or the holding power of the sheet's fasteners.

Hollow wall anchors should be used as a last resort and with a grain of salt. Use common sense, and never use them for things that move. Since shelves don't move, but the contents do, I'd not rely heavily on anchors except in rare cases where studs aren't available for one part of the shelf.