When touring, our all-up tandem weight is ~380 lbs. We use 36 spoke wheels with double-butted 14-15 spokes and either Deep-V or Kinlin 279 rims. The deep rims are important. We have toured on cobblestones and have never even needed to true a wheel on tour and have never broken a spoke. We could probably use 32 spoke wheels just fine. The old advice to use more spokes was based on the shallow rims which were in vogue until fairly recently. Then deep rims came on the market.