Similar to character limits for Alphanumeric fields, The amount of decimal places required in a Numeric User-Defined field must be set at the time the field is being created.
Think about the information that this field will capture, for example, if, you are capturing the amount of kids a client has, no decimal places would be required, however if you are capturing something like an exchange rate, you may want to 2 or more for greater accuracy.
The Maximum amount of decimal places allowed is 10.
Once the field is created, you CANNOT adjust the amount of decimal places for a Numeric field.