Yeast ratio

2 yrs. ago I made applejack for the first time, & it came out great. I was nervous, so I followed the recipe slavishly. It called for a ratio of 1 gal. juice, 1 gal. water, 4 lbs. brown sugar, and 1-2 packets champagne yeast. The guy who sold me the yeast says 1 packet should be good for 5 gal. It was great last time, & I am afraid to mess with success. Maybe it was so much yeast because of all the brown sugar? What are the pros & cons of the yeast ratio? What can go wrong if I use too much or too little?

Thanks, guys!