USPS tracking numbers are 22 digits.
The first five digits comprise the service code. The first 2 numbers tell where/how the label was printed.
- 91 (legacy/other)
- 92 and 93 Commercial Mailers
- 94 Electronic (From the users device)
- 95 Made at the Post Office
The next 3 digits state the type of service the package is being sent.
- USPS Tracking: 001
- Priority Mail: 055
- Certified Mail: 073
- Registered Mail: 088
- Signature Confirmation: 021
USPS has published a full list of services in USPS Publication 199
The next 2 numbers indicate the sender. And the last 6 numbers are the account number given to the business or individual if they have an account with USPS or if not it is randomly given.
USPS delivers approximately 20.5 million packages daily. With nine undefined (to my knowledge) numbers remaining that gives us 1 trillion combinations, or 48 days worth of unique tracking numbers.
Of course this breaks down if USPS assigns tracking to individual accounts/services, etc.
But essentially, yes to recycing tracking numbers,