How do they know that 1 in 5 people has herpes, if 90% of people with herpes don't even know they have it?

Herpes Blood Tests are generally used in cases where no visible symptoms are present. A blood test works by detecting the presence of herpes antibodies. There is a possibility that the virus will not show up in a blood test, and a positive result is not always indicative that a person has genital herpes. Firstly, after the first exposure to herpes, a person may take several weeks to develop the antibodies that the test looks for. Usually, it takes two weeks to three months after exposure to herpes for antibodies to appear in the blood. Some blood tests detect antibodies sooner than others. However, once antibodies are found they remain in the body for life. Secondly, blood tests cannot tell the difference between the two types of herpes, HSV-1 and HSV-2. For this reason, anyone seeking an accurate diagnosis of genital herpes must be sure to get a "type-specific" serologic herpes test, which can accurately distinguish HSV-2 from HSV-1 antibodies. Most commercially available kit assays currently cannot make this distinction despite their claims.

They take samples of the population. For instance, they could sample blood from blood donations or have a blind study where people sign up to donate blood. Just like any other science, they have to go based on the law of large numbers instead of by the entire population.