Pick one. Preferably in the late rounds. I'd consider that after round 10. Remember, defenses are fairly unpredictable because you are getting points from an entire unit of 11 starters and their backups. You're not just picking based on one player's output.
It is reasonable to take a D/ST earlier (unlike kickers). Keep in mind that the earlier you take a defense, the more value you miss out on at key positions.
Look up how your DST's are scored and determine if there is value to taking one earlier.
Round 10 is normally the earliest you should ever take a D/ST (assuming you have fairly standard scoring).
Draft only 1 D/ST. But if you need to, play matchups and rotate some D/STs.