When making a pot of mashed potatoes, do you put in half a stick of butter or a full stick?

I haven't cooked in 6 years so, I forgot.
My friend's wife just died a bit over a month ago and he mentioned that he doesn't know how to cook so, I was gonna write him a few recipes I know but, I forgot how much butter to put in the potatoes.

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I peel them, slice them into quarters or 8ths because, the smaller they are makes them cook faster.
Drop the slices into a pot of salted water. Cook them for about half an hour (I think). By this time, the roast should be at least half done so, pour the potato water into the roast pan Dump the rest and drop the butter in with the lid on. After a few minutes, add the milk and mash the potatoes, then whip them a bit.
When making a pot of mashed potatoes, do you put in half a stick of butter or a full stick?
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