There's no easy answer when it comes to how many amps is a good jump box because it really depends on the situation. If you're just using it for a quick jump start, then you might be able to get away with a lower amp unit. However, if you're stranded in the middle of nowhere and need to run your car's engine for an extended period of time, then you'll want a higher amp unit.
Generally speaking, most jump boxes have between 400 and 1000 amps of power. This should be enough to start most cars (assuming the battery isn't completely dead). However, if you have a larger vehicle or one with a diesel engine, you may need something with even more power.
The best way to determine how many amps you need is to consult your car's owner manual. It will typically list the minimum amperage required for starting the engine. Alternatively, you can also ask an automotive specialist for advice.