Sex addiction is a common misconception. Sex addicts, in other words, are not simply people who crave lots of sex. Instead, they have underlying problems -- stress, anxiety, depression, shame -- that drive their often risky sexual behavior.
Experts prefer the term "hypersexual disorder," rather than "sex addiction." By either name, it's about people who keep engaging in sexual behaviors that are damaging them and/or their families.
As examples men who spend half their income on prostitutes, and office workers who surf the web for porn despite warnings that they'll lose their job if they keep it up. They can't stop what they're doing. They're preoccupied; their brain just keeps going back to it. It often leads to loneliness and isolation.
There are no reliable estimates of how many people have the disorder. Some studies suggest that it's more common in men, and gay men in particular, than women. Those people are going to seek medical help.