Insert a comma between a list of taxonomy terms

I'm trying to output a list of taxonomy terms on a page, inserting a comma and a space between each of them (but not for the last term). I can get the current count, but how do I compare that to the total number of terms in the list to create a conditional to skip the comma for the last term?

{{ relate:services }}
     {{ title }}
     {{ if count < variable_that_holds_total_count }} 
          ,&nbsp;
     {{ /if }}
{{ /relate:services }}

I've looked in the documentation but I didn't find anything. There must be some way to do this with the template tags that I'm missing.