According to the label, there are 21-22 mini marshmallows per cup.
If your recipe asks for 1 cup of small marshmallows then put them in until they make one cup. You can LIGHTLY PRESS (not too much) until you get the one cup.
Kraft foods emailed me an answer on this and other marshmallow conversions this is what they said- For JET-PUFFED Marshmallows: Conversions for Marshmallows and Marshmallow Creme: 7 oz Marshmallow...