It depends on how much caffeine is in your cup. Rather than measuring the size of a cup, I would go by the amount of coffee, as in the weight of the beans, used to brew it. The actual amount of caffeine is not as easy to measure, and even for the same kind of beans, there is natural variation.

For a traditional Italian espresso, about 7g of coffee beans are extracted. For a third-wave double espresso, it's usually 18g or more.

In my opinion, 10x7g is a lot. 2x12g is more than enough for me.