How much voltage/amperage can a NPN transistor take?
Saying you have a "Transistor" is like saying you have a "vehicle", and you're asking us what your vehicle's towing capacity is.

Basically, there are many, many, many thousands of varieties of transistors.

Anyways, if you follow the "datasheets here" link, the NPN transistor included with your kit is a SS9013.

This particular transistor is rated for a maximum (collector-emitter) voltage of 20V, and a maximum current of 500 mA.

This means that you can switch a (small) 12V device with the transistors you have. However, if the item you're switching needs more then 500 mA, you're going to either need a bigger transistor, a relay, or some other switching device.  

If you can give us more information on what device you're hoping to switch, we can probably help you determine if the NPN devices you have will work, or if not, what devices/relay/whatever you should use.