How are brushing teeth once and brushing teeth twice a day different?

Not a lot: You mouth will feel fresher more often. A Fluoride toothpaste will add to the decay resistance. Proper flossing and brushing once a day is far better than brushing 2 or 3 times a day.

Better oral hygiene: Proper brushing and flossing after every meal is ideal. The closer you adhere to that the better. I emphasized proper and after as many people do a very quick job and not necessarily properly. Also many brush and then they eat. It's better to reverse the order. Ask your dentist or dental hygienist to go over the techniques with you. It will pay off in the end with less cavities & healthier gums.