Hi, We are planning to use sharded replica-set in production and would like to know if there are any issue with just running a single shard? As our data grow, we will add the second. It seems to run fine in our test environment.

Your document specifically mentions to run 2 shards: http://docs.mongodb.org/manual/core/sharded-clusters/#infrastructure-requirements-for-sharded-clusters

The reason why we're doing this so that we don't have to convert replicaset to a sharded setup later on as the coding requirement is different for findAndRemove/findAndModify (shard key requirement for these calls) and deal with the shard key aspect at a later time.