system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are tracking numbers created and how long will it be until UPS, FedEx and USPS run out?
gpt
Tracking numbers are randomized numbers and letters, typically in a HexDec system that allows for 16 character numbers, and are typically like in UPS's case, 18 characters long following the 2 character preamble.. This equates to a combination that is as follows:

18^18 = DAMN! or 39,346,408,075,296,537,575,424. That alone give you so many different numbers, that it is impossible to run out of tracking numbers. 

For USPS, they use a 20 digit Number system with the first 4 numbers predesignated.

so 4 + 10^18 = 1000000000000000004. That is exactly 1 Sextillion combinations.

Tl;DR; we can track everything.

edit for the UPS snob at the bottom.