The answer at WikiAnswers
states: "as many as that teenager's body wants to have. There is no other answer. With your body filled with testosterone - growth hormone - and your penis rubbing against underware everytime you move, you are going to have frequent erections." Other names are woody, stiffy, or "hard-on".
All men experience erections while they sleep. You will usually have four or five each night, though you won't notice most of them because you are asleep. Unbelievably the average nocturnal erection last between 20-30 minutes. If you do the maths the average man has a “hard-on” for between 5-6 hours when asleep.
Most healthy guys wake up in the morning with a massive erection, often known as "morning glory". That is the first of whatever will follow during the rest of the day. Testosterone levels peak in the early morning hours, which causes spontaneous dilation of the arteries leading into the penis and constriction of the veins leading out of the penis.
The proper scientific name for the early morning erection is 'Nocturnal Penile Tumescence'.