To start of with, you should try flossing just once a day, but when this becomes part of your routine, you should start flossing twice a day. If you started flossing twice a day at the beginning, you would most like find it difficult to keep going so you would just give up.
Once a day is actually optimal. Constant flossing can irritate the gums' soft tissue and cause inflammation and bleeding. Over time, flossing too vigorously can also wear down your gum line and tooth enamel. So floss your teeth gently.
The only exception to the once a day flossing would be after eating something sticky, hard, or stringy. That can get stuck behind your teeth, but just using a toothpick should be fine.