You can adjust the number of vanilla pods used depending on how much you want to make. Vanilla extract will keep indefinitely, so it is quite alright to make a large batch and store it for a long time.
Split the vanilla pods lengthwise and scrape out the seeds. I keep the seeds in a separate container and use them in recipes.
Put the pods in a glass bottle or jar and top up with vodka. I like using my old maple syrup bottles, solely because they are cute! When I want to make a bigger batch though I will use a large jar, such as one that had pasta sauce in it - that size.
Store in a cool dark place for about 6 weeks, shaking every now and then.