I always regard 6 as being the official figure.
If your glasses are so big that they take 1/4 bottle then they'll probably lose the sparkle in the champagne before it's finished (or you could just drink very quickly!). My solution is to keep the bottle right beside me so I can top up as necessary.
If your glasses are so small that it takes more than 6 glasses to fill a bottle then you're very mean and what are you doing drinking champagne anyway (unless you got it as a gift)?
Remember, to open it without spilling a drop (for which crime you should be locked up for life!), DO NOT SHAKE the bottle. Peel off the foil. Hold the bottle at a 45 degree angle. Remove the cage. Hold the cork in one hand and the bottle in another. Gently turn the bottle (not the cork) till you feel pressure in the hand holding the cork. The cork will then pop out into your hand with a very satisfying "POP" and everyone around you will be really impressed!
BTW, practice makes perfect at this!