Put the potatoes in a large saucepan and cover by about 2 inches with cold water.
Bring to a boil and cook, partially covered, until the potatoes are very tender, 15 to 20 minutes.
Drain the potatoes into a colander and return them at once to the pan.
With a potato masher, mash the hot potatoes with the milk.
Add the chiles, and sour cream.
Mash until fluffy.
Add more milk, if needed.
Stir in salt, and taste.
Adjust as needed.