Good question, Stealth. Sadly, despite the number of DVD players I own, I've never watched more than two at the same time. There has been at least one moment in time that a DVD was playing on the office PC, the living room DVD deck, and the master bedroom DVD deck. There were three people (a brother was staying with us) each watching a different movie in different rooms.

And thinking more about the original question, I'll have to add "Any Notebook or desktop PC on my home network, even if they don't have a DVD drive, can still play DVDs backed up onto a Hard drive shared on the network." So since both desktops have DVD drives of their own I won't count them, but two notebooks that don't have DVD drives can access the DVD files and play them, so plus 2 for 21. OMFG!