Doesn't it depend on your SG, and what kind of beer or cider you 're making? I just put together a cider with a SG of 1.090 and used a packet of Red Star Montrachet. I'm thinking it may not have been enough, and this is a 2.5 gallon batch. The way i understood yeast and sugar is that the more sugar you have, the more yeast you need to attenuate it down. A 1.040 beer wouldn't need as much yeast as a 1.090, or maybe not necessarily NEED it, but be able to use it. I brewed a 1.100 belgian quad, used a smack pack that I made a starter out of, and pitched a WLP500, just to be safe. It's been 25 hours, and the things been going crazy. blowing off kreusen, and bubbling up like a machine gun. I added the extra yeast because I raised the SG and didn't want to get stuck with a batch of beer that wasn't fully fermented. I'm only on my 4th batch of beer and first of cider, so maybe I'm not so well informed on the activity of yeast, but this is how I thought it worked