Not clear from your question exactly what you're trying to achieve.
The CRM email subject tracking tokens are not necessarily unique. They will be unique to that conversation within your CRM organisation until the numbers run out and it starts over.
Every record on CRM has a truly unique GUID which should be used if you need a unique identifier. If you don't like the long and complex format of the GUID (your unique reference will be human facing) there are a number of automatic numbering solutions available for CRM which will generate friendlier unique ids for records.
Hope this helps, Tony