How many Bluetooth devices can you connect to the iPhone 4s at once?

One I believe although a low-power piece of equipment, such as a heart monitor which is compatible with that BT profile, might be able to connect simultaneously with a headset.

At least two, possibly more as long as they are using the same bluetooth profile.

I use bluetooth headphones with my Wahoo bike sensor every morning.

I can also use my bluetooth headphones with a bluetooth heart rate monitor. I do not think I've ever used all 3 together but there is no reason it should not work.