In the MongoDB documentation for auto-sharding it says: "Sharding is performed on a per-collection basis. Small collections need not be sharded."
Our business has many databases (~100), with many small collections (~30), each with a document count of 1 - 3000. Our DB system is looking at approximately 100,000,000 page views per month.
In that scenario will sharding ever activate since the collections are never big enough even though the DB usage and site traffic is certainly high enough to require load balancing. From the docs I can't seem to find a clear answer.
Whether it makes sense to shard depends a little bit on whether you have mostly writes or reads to the database. Sharding is primarily used for write-scaling, but if you are not doing a lot of writes, then simply using replicasets with "slaveOkay" for the reads might work just as well.
From the numbers that you provided you seem to get about 9 million documents, but are they large documents? If they easily fit in memory, then there is most likely not even going to be a need for replicasets besides for failover capabilities.