When I see the questions list, I see the questions with number of answers. Some of them (the numbers) are in green background and others are against normal white background. What is the difference? If I take that as the approved/accepted answer by the OP, can he/she accept more than one answer?
The background is green if OP has accepted an answer, and gray if OP hasn't. In some lists the green number is the score (upvotes - downvotes) of the question. (For example, linked questions, related questions and shorter list in user profile.) In some lists the green number is the number of answers. (For example, list of questions, front page or questions tab in user profile.)
No, OP cannot accept more than one answer, that defeats the whole purpose of it.
Under the home page tabs, the box displaying the count of the number of answers is shaded green when there is an accepted answer. Only one answer can be accepted by the OP.
It looks like in this example (I will add link to the question as well):
Or in some lists the question is shown without the preview of the beginning of the post:
You can see there two numbers. The number 41 is score (number of upvotes minus number of downvotes). The number which is highlighted in green is number of answers.