Q: How many times a week do i need to train?

Well that all depends on how much time you have and how often you surf.
If you are a weekend warrior and only get to surf at the weekends, then i would aim to train at least 2 x during the week with strength and conditioning training. Circuit training is an excellent form of endurance training for surfing with exercises that look and feel like surfing during your circuits, i.e., press ups, dips, lunges and torso twists etc...

If you can fit interval cardio blast (swimming, running, rowing) to boost cardio fitness and/or yoga to improve flexibility, posture, breathing and focus, in there as well 2 x a week then that is even better.

If you are fortunate to live by the beach and surf as much as seemingly possible, then lets face it, you are not going to want to do anything else if there is a wave to be had. However I would definitely recommend yoga and flexibility training a minimum 2x week to counteract the amount of surfing you are doing. And on the lull / flat days it really is advisable to keep up your fitness training with strength and endurance conditioning as well. (it will make a huge difference when there has been no swell for the week and you get back in the water).