I understand that you wish to ensure that the number of something-delimited items does not increase nor decrease.
I cannot come up with any RegEx search that can do that so the options with Xbench would not be a direct approach:
Option A. Write an Xbench QA plugin (documentation and sample code in GitHub) that especifically tests that. It is a high difficulty item because it requires programming skills in your team.
Option B. Make a copy of the input files and do a global change in source and target of the delimiter by something tag-shaped, for example
<comma>. Then the Xbench tag mismatch check would be able to catch if there are extra or missing commas in the sequence. You can also seach for appearances of two
<comma> tag in a row. This is a much simpler approach, the skills required are basic scripting skills in your team.