When Rift first came out, Triond announced that there were over 1 million pre orders for the game. However, how many people play Rift now?
This is actually quite a difficult question to answer, as there are no exact stats floating around online. It seems as if the player base could potentially be shrinking, as some server/shard merging is going on. That is usually a sign of a dwindling subscriber base in MMORPG games.
On the other hand, that could just represent Triond working towards increased efficiencies and economies in the Rift project, thus reducing the risk of subscription fee increases (which would honestly kill the player base IMO)
The short answer is that nobody (apart from the game developers themselves) are quite sure how many people play Rift.
You want my opinion? Enough people play it to ensure that you are going to have a good time, so if you don't already have a copy get one now!