It really depends on how your bow balances.
Most who use just one, are putting it opposite the sight to offset the weight on one side of the bow, in addition to providing a little back weight.
If you are shooting 2 V-bars, then one is normally weighted heavier than the other (see above) and will tend to give you a little more back weight overall.
It really comes down to personal preference and your shooting style.
The recurve guys use two because a recurve with a target stabilizer is so front heavy they need some back weight to keep it from falling away too fast.