Typically 4.
1. Travel kindle (easy plots due to interruptions)
2. Editing kindle (reading and editing book I scanned)
3. Whispersync book
4. Any other book - usually long one
Audiobooks (typically 2)
1. Travel ipod
2. Home ipod.
I could get by with 1 ereader and one Ipod, but I don't have to.

I'm better than I used to be - it used to be 8 books and 3 audiobooks.