How often does aspiration happen and exactly how does it happen? and why? how to prevent it? and can acid reflux cause it?

Depends: It's entirely dependent on the individual patient. Acid reflux causes it. Indeed you have to have reflux, technically before it can be aspirated. Predisposing factors are lying down with a lot of food in the stomach, age, diabetes, alcohol and sedatives and narcotics and other drugs, and any deformity of the structures closing off the upper part of the stomach like hiatal hernia.