How many DJI Goggles can be connected to DJI Phantom 4, Pro, Advance, by
a) Wi-fi
b) HDMI
c) USB

Is there any DJI drone can be connected to 8 DJI Goggles?

If 8 people are watching, you may be better off connecting to Facebook Live or Youtube Live and let them watch that way. There is no device that will hook up to 8 DJI Goggles. One goggle would be needed to hook up by any one of the methods provided as as Wi-Fi or USB. You may be able to get a second set hooked up with an HDMI but more investigation needs to be made into that point.