How common is low testosterone in men?

Depends upon age. As we age, we tend to produce less & less testosterone. So the frequency of low testosterone increases w/age. However, the number isn't as important as how you feel. I wouldn't recommend checking levels just to prove a point. Rather i would consider checking if you were complaining of hypogonadal symptoms. Likewise, my goal would be your clinical improvement, not just a number.
Very! Low testosterone is a statistical derivation. What's more important than a number is how someone feels. With that said, more & more men meet laboratory standards for low testosterone as we age. But the answer to what should be done is very different from what can be done. Lots of testosterone supplement options are available but only those who feel hypogonadal should get treated.