This 2017 report titled WNTI Standard for UF6 Cylinder Identification has a wealth of information about that topic, the complexity of which, in my opinion, this portion captures:
Facilities that regularly use model 30B and 48Y cylinders include conversion, enrichment, fuel fabrication and depleted UF6 (DUF6) conversion facilities. Since these facilities are typically not at the same location, the process for handling, processing, tracking and reporting on the nuclear material contained in these cylinders can be rather complex. Today, there are approximately 20,000 cylinders in active circulation and greater than 100,000 cylinders in long-term storage.
Here’s a useful chart: