Is it healthier to have 1-2 bowel movements a day, or 3-4 in adults?

Everybody different. However, in the perfect world the answer would be one or two. As americans we eat so much processed food that there is not enough bulk to allow proper emptying which makes us have air pockets between stool sections. We then have a bowel movement to the next air pocket. This leaves bowel content to stay against our bowel wall causing inflammation.
Either. Everyone is different in terms of normal bowel frequency. For some it's 1-2 a day others it more or less.