Over 95% of people fall below 250 mg a day of EPA and DHA, the omega-3s from oily fish, according to research from the Linus Pauling Institute. That shortfall is the real story, and it hides because "omega-3" actually covers two different things.
One is ALA, the plant form found in walnuts, flaxseed, chia, and common cooking oils. The other is EPA and DHA, the marine forms from oily fish, which your body puts to most direct use, supporting your heart, brain, and eyes. The catch is that your body converts ALA into EPA and DHA only very inefficiently, so eating plenty of the plant form doesn't reliably top up the marine forms. Someone can look like they get enough omega-3 overall yet still be well short on the part that matters most.
The numbers show it. A common target for total omega-3 is around 2.2 g a day on a roughly 2,000-calorie diet, much of it ALA, and that broader goal is met fairly often. The smaller marine benchmark of 250 mg EPA and DHA is the one most people miss, because it really only comes from eating oily fish with any regularity.
The simplest way to close the gap is oily fish like salmon, mackerel, and sardines a couple of times a week, with walnuts, flaxseed, and chia adding the ALA form alongside. There's no established upper limit at a specific figure, though very high supplemental doses are best discussed with a doctor.