When making a bechamel, I like to think of a 2:2:1 ratio, which can easily scale up if you are serving a large group
2 tablespoons flour
2 tablespoons butter
1 cup whole milk
Melt butter in a heavy bottomed sauce pan and add flour. Cook and stir constantly for about a minute or two, or until flour starts to smell 'toasted'.
Slowly whisk in milk, stirring constantly so it doesn't start to stick to the bottom of the pan. Cook for about 2 minutes, or until it thickens slightly and can coat the back of a spoon.
Season to taste to salt and a bit of nutmeg if desired.
If you'd like to make a cheese sauce, simply add a cup of grated cheese of choice into the sauce once it's thickened and let the cheese melt off the heat. Whisk until smooth and enjoy!