The chances for injury are pretty high. This depends on how hard you ride, and if you even know how to ride.
The chances of death are a lot less, but anything can happen.
Get an ATV. I feel a lot safer on my quad. I also find myself getting hurt way less when falling off the quad as opposed to the dirt bike.
Everyone falls here and there, it's just a matter of how they crash and how fast they are going.
If you don't know how to ride, you will most likely sustain several mild injuries.