Why do I wake up multiple times per night?

Depends. Normally, people wake up 1-2 times per night depending on age. There may be many reasons ranging from sleep environment (noisy, uncomfortable bedroom), unhealthy diet (excessive alcohol, large fatty meals before bedtime, nicotine or caffeine), certain medications, stress and psychiatric problems and other sleep disorders such as obstructive sleep apnea. If becoming disruptive, would see a doctor.