To create a number entry field select "Number" from the "Type" dropdown box. You can choose how many decimal points should be used, the width, and whether you want it preceded by a US currency symbol.
Deselect the "No Default" checkbox if you want to enter a number that will be automatically entered if this field is skipped when a new record is created. Then you can enter the default number in the "Default" box to the right of the "No Default" check box.
"No Default" is the default for all of the UDFs that have the checkbox.