InfoPath Dev

if my user types in some info into a drop down, depending on that info i would like to auto-populate a text box with a number.

I cant use rules, because there are too many selections and that would get very aggravating.

Whats the easiest way to do this via code?

Thanks so much!

Type in info to a dropdown?  Do you mean selects from the choices in a dropdown?  What is the logic for how selections correspond to a number?  why not make the value for the display names equal to whatever number is required?  That way, you already know the number the moment the selection is made, and all you have to do is set the default value of your text box to the value of the dropdown.  It will show the number (value) after the dropdown selection (display name) has been chosen.  You can either do this directly in the form's data source in the control, or you can have a sharepoint list (or other database source) with a table and two columns - one for the selection and one for the associated number.