We add the "s" to these numbers when we aren't being at all specific about the number.
There are hundreds of people in the room.
There are three hundred people in the room.
There are about three hundred people in the room.
There were thousands of birds on the lake.
There were four thousand birds on the lake.
There were approximately four thousand birds on the lake.
There are millions of insects which live under the ground.
There are two and half million insects which live under the ground.
There are about two and half million insects which live under the ground.
There are billions of people in the world.
There are six billion, eight hundred and twenty nine million, six hundred thousand people in the world.
There are nearly seven billion people in the world.