Is the amount of yeast directly proportional to the alcohol content?

Is it directly proportional to how strong the beer will come out, or is it a little more complicated than that?

The amount of yeast has little to do with it. The amount of fermentable sugars is what will determine the alcohol content. You can get a good general idea with a hydrometer.

If you haven't done so already, I can't stress enough to read this free online book. Palmer explains things really well. How to Brew - By John Palmer

Thanks, I was confused. Would you say then that the higher the OG, the more alcohol %?

The greater difference between OG and FG the greater the abv
estimated ABV = (OG - FG)*131

ant8344 said:

Thanks, I was confused. Would you say then that the higher the OG, the more alcohol %?

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Yes, the higher OG the higher the alcohol potential. What it finishes at can depend on the strain of yeast, but not really the volume used because the yeast always multiplies itself to a certain concentration before it will start fermenting the wort. Then what it finishes at (the FG or final gravity) will tell you how much alcohol was produced, using the formula Deacon stated.

Though to some degree the amount of yeast pitched can influence the final gravity, which is why larger starters are recommended for high gravity beers.