How fo you know how much yeasr?

Wanting to improve my beer kits by using better yeasts, smack packs etc ,
Looking on mr malty etc etc
Gives an amount proportional to O.G how do you know how much to have in stock or prep repaired if the wort isn't made therefore not knowing O.G

You make the recipe in a brewing software and/or look at the recipe OG if you are following a kit and type in the estimated amount. Unless you generally get drastically different results from these calculations these are a good starting point.

Depending on what type of beer you are going for it is generally much easier to underpitch than to overpitch.

I now use beersmith 2.1 and it told me I needed more yeast.... but I didn't listen...

The second batch I did... an IPA I am sure I underpitched so the ferment is taking a bit longer than it normally would have (sitting at exactly a week now) I checked it on Sunday and it was down from 1.078 to 1.030. Figure I'll check again later this week and see where we are... then hope to bottle of thanksgiving break.