What happnes if you remove the literal quotation marks from either end?
Wildcards are not supported by the PatternCount function.
You could use:
PattternCount ( yourText ; "ref=ntt_athr_dp_pel_" )
But that won't work if there are instances of ref=ntt_athr_dp_pel_ that do not have the number and that thus need to be excluded from the count.
If you have FileMaker Advanced, you might search one of the custom function web sites to find a custom function you can add that provides the text searching capabilities that you want.