Polyunsaturated fat is best understood as a family rather than a single thing, because it's the umbrella that contains both omega-3 and omega-6. Like monounsaturated fat, it's one to favour over saturated and trans fats, and replacing them with it is associated with a better cholesterol profile over time.
What makes the category worth understanding is its two very different members. Omega-3, largely from oily fish and some plants, is the one most people fall short on. Omega-6, from vegetable and seed oils, is abundant in modern diets and rarely lacking. So "getting enough polyunsaturated fat" isn't really the useful question; the balance between its two halves is, and each has its own page.
The broad sources are fatty fish, walnuts, flaxseed, sunflower and soybean oils, and seeds. On a roughly 2,000-calorie day, total polyunsaturated fat works out to around 18 g, scaling with overall intake, and there's no upper limit set for it. For the part that matters most, the split between omega-3 and omega-6, see those two pages.