Hi,
As far as I know you can't define "number" form element on a datapage, I suppose that your field is set to text form element even if your datatype is number in a table. However, you will not be able to submit characters, an error message should appear once you hit "Submit"
I use script from this post on my datapage. This script allows entering numbers only in a field.
Another option - you can create an html form and submit data to caspio: http://howto.caspio.com/tech-tips-and-articles/advanced-customizations/how-to-convert-existing-html-forms-to-work-with-caspio/