Bear with my convuluted way of thinking...
Seems like you need two batches, or six cups of icing, to do 24 cup cakes with the 1M tip. So each cupcake is 1/4 cup of icing.
So 250 cupcakes times .25 equals 62.5 cups of icing.
62.5 cups divided by 3-cup batches is...20.8 batches.
Does that compute??
Alot. 
If you are making big swirls of icing, I guess if it were me I'd probably make four 6qt kitchenaid bowls full of icing and then see from there.... that would mean each bowlful would swirl just over 60 cupcakes (you hope). For me I can get a quadruple batch of Wilton half shortening/half butter buttercream in one 6 qt mixer.
That's what I would guess... you can always refrigerate or freeze icing for later if you have leftovers, so buying extra ingredients wouldn't hurt. I'd probably buy enough ingredients for six 6qt bowls of icing....
Bear with my convuluted way of thinking...
Seems like you need two batches, or six cups of icing, to do 24 cup cakes with the 1M tip. So each cupcake is 1/4 cup of icing.
So 250 cupcakes times .25 equals 62.5 cups of icing.
62.5 cups divided by 3-cup batches is...20.8 batches.
Does that compute??
a quarter of a cup PER cupcake, really? I would think half that, or am I just kidding myself?
Melissa
Kitagrl's estimate adds up to 48 cups, dividing that out would be 1/5 cup per cupcake, so a little less than my estimate.
So, somewhere between 48 cups and 62.5 cups?
Let's go back to "a lot!"
LOL start making icing, girl.... 
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