The maximum number of video windows per call is (12) and can include a mix of PC, MAC and mobile participants.
If you own credits or phone minutes you can dial-in an additional (6) audio-only participants; they can hear the audio on the call but can not see anything.
Here's some other info:
If you would like to learn more about our different plan and credit options click here: http://www.oovoo.com/Buy.aspx