Q:

Make infopath accept Number only

Is there anyway for a textbox in infopath form to only accept numerical values without any back end programming? 
Thanks

A:

You can use the Data validation built-in as the following
Try this: 

Double-click the field 
Click Data Validation at the bottom of the dialog box.  
Click Add 
Select Does Not Match Pattern from the second drop-down box.  
Type the following custom pattern into the third drop-down "\d*".  
Type the message you want the user to see in the short and detailed error message fields.  
Select Dialog Box Alert from the drop-down.  
Click OK three times.  

Ref: Numeric only fields
For more details about Data validation check 3 Ways to validate data in InfoPath

