Watts = Volts x Amps
At 12v, 15amp fuse allows 180w current -- which would be a lot of chargers plugged in!
More of an issue is the likely short-circuit potential with lots of adaptors and plugs lying around. Some of these multi-way 12v adaptors are really nastily made, and offer little protection to keep 'foreign' metal objects' out (eg loose change, foil wrappers etc)