Counting the total number of responses of a contact in XM Directory

Hello everyone, 
We have a respondent pool which we will distribute surveys occasionally. 
We want to count the number of surveys each respondent attended to so that we can send them a thank you e-mail when their total number of responses reaches certain points (10 and 20). 
I guess we need to be creating an an e-mail trigger or action based on an embedded data field. Which is created as "TotalResponseCount" when contacts are imported from a .csv file to XM Directory. 
I am trying to update this field at the end of each survey by using math operations. 
I add a screenshot below showing the Survey Flow I am working on to count the total survey responses.


But this operation does not seem to work. The data field stays the same after I complete the survey several times with the same credentials. What am I doing wrong? Is there a better way to do what I am describing here?
I am relatively new to contact management so I would appreciate any help for completing this task

Thanks!