If you make them following the directions on the box you'll get 24 regular size cupcakes. Fill them about 2/3 full. I use an ice cream scoop so that each one gets the same amount.
I also use an ice cream scoop to measure my cupcake batter. It says on the back of the box that it makes 24 cupcakes, but I usually end up with 20-22 cupcakes. HTH
I do my first pass with a #16 scoop, then I go back over all of them with a #40 scoop.
Now...I've tried using those little nut cup thingys that are kind of popular, and those things will flow over like a volcano if you go over half full. I've destroyed thousands of cupcakes figuring out just how much batter to use in those! 
Recently I my son asked me last minute for cupcakes so I used a doctored Betty Crocker. I did used a small gravy ladle to fill the cupcakes and they baked up perfectly level with the cupcake wrapper. It came to 40 cupcakes!! He only needed 20 , some leftovers went to work with my husband and the rest went to my hips : )
If you make them following the directions on the box you'll get 24 regular size cupcakes. Fill them about 2/3 full. I use an ice cream scoop so that each one gets the same amount.
This is what I do when I make cupcakes too. 
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