To keep your teeth clean and healthy and to prevent gum disease you should floss at least once per day between tooth brushing.
Some people including myself floss their teeth twice per day such as after a meal such as dinner and then again before going to bed.
I used to only floss at night before bed and after taking a shower but I started flossing my teeth after I have dinner and then floss between my teeth once again before bed.
I've noticed my teeth are much healthier and have stayed cleaner as a result of flossing twice per day.
I brush my teeth twice per day and also floss my teeth twice per day as well.
As long as you floss once per day that's usually good enough but flossing more often is not bad either.