Of course the answer is that you can never buy too much as you will use it for something later. Still, economics comes into play sometimes and you need to consider how much glass is enough for a given project.
I find that the larger the pieces in the project, the more glass I need. There is greater wastage with large pieces than in a project with many small pieces. I always cut my large pieces first, as this leaves cullet for smaller pieces, thus reducing the wastage.
If you are using glass that has a definite pattern or flow to it, you need to plan on a greater wastage factor than if you are using plain glass or textured glass with no particular direction.
The stage you are at in your cutting will also have an effect on how much glass you need. At earlier stages you will have more unexpected breaks than later on, so take that into account too.
Some people report that they can manage with one third more area than their project, others one half more. I find that I usually need twice the area to have enough glass to complete the project.