There's enough for at least 5 gallons of cider. If you make just a gallon probably half the packet will be enough but it won't hurt if you chuck the lot in.
There isn't really a cider yeast, most of them are just beer yeast with an artificial sweetener in them. We've chosen this one because it sets off fast, you need that because the apples start fermenting as soon as you press them. It settles down nicely as well.
We don't think you should be adding artificial sweeteners to your cider so there's none in this yeast!!
The old books recommend Champagne yeast, it does work but starts a bit too slow and ferments out very dry. This is better for normal ciders, if you're going for high alcohol and adding loads of sugar then use the Champagne yeast