I currently have two with my DVD changer and my cable box (DVI actually). I will be adding a PS3 in November, and that should be it for a long while.
For HDMI inputs that is a simple one... one. Once again, actually DVI. On the projector? DVI - Non-HDCP compliant (go figure)... On the bedroom plasma? One! This time it is actually HDMI.
When the Blu-ray player shows up it'll be time to start shopping a new projector out that is 1080p/24 compliant and with HDMI 1.3.