One at a time. Are you sure you have plugs and not electrodes? With new technology, you never know. Look in your manual or see the dealership.
The newer Isuzu does not have spark plugs but coil packs and are controlled by the on board computer system. There is no need to replace them unless they fail and your check engine light will come on. This vehicle is OBD-II compliant so can be checked with a small diagnostic tool that is offered at many part stores. Hope that they don't fail as they run about $150 a piece, a far cry from a $7 spark plug!