How common is pulmonary embolism in general? and in 20s?

See below: More than 300,000 people each year have deep vein thrombosis or a pulmonary embolism. Depends on various factors, i.e. anything that makes you more likely to form blood clots increases your risk of pulmonary embolism. Getting PE in the 20s is unlikely unless there are underlying predispositions, such as hypercoagulability factors, such as sickle cell disease, certain protein deficiencies, etc.

Quite uncommon: especially in your 20s unless there is a predisposing factor