TM transactional documents do have numbers, which are typically drawn from an internal number range. This number range is finite, and the status should be monitored.

This note describes how to monitor the current status using the report RSNUMHOT.

Important: As most other SAP number ranges, the TM number ranges are defined as rolling, meaning, that this needs to be considered in the selection. BTW: Rolling means, that the number range starts again with the start number, once the last number from the numberrange is consumed. As we do check for duplicate numbers, the old documents  must have been archived, when the Rolling applies.

But of course, all of this should rarely be relevant for you. The number range should be defined large enough for the expected usage!