Activly working: just one.
But I have an idea for the next book simmering away in my brain. I don't write anything down on it yet, I just let it grow momentum in my mind. I'll notice things that will be good to check out during the research phase, settings worth remembering, etc, but I don't take any notes while working on the WIP.
I reckon that if it's an idea worth remembering, I'll remember it. And my subconscious mind will work on it in a way it would never be allowed to if I wrote things down as I find that the written words tend to trap me. So all though I'm really anxious and eager to get started on the next project, I almost enjoy the fact that I have a WIP to work on first, so that it can fully develop within me before I set out. It's too easy for me to get excited while writing and thus skipping important steps along the way.