No free number range found for document type &1

What causes this issue?

The document type is used for a mass processing that is run in several
parallel processes. Each process requires a separate number range. The
system has been unable to find a free number range, the reason being
that there are more processes active than the permitted number ranges
available.
A number range that is at least 98% exhausted is no longer used to
ensure that there is no surplus of number ranges for mass processing.

System Response

Processing terminates.

How to fix this error?

Assign additional number ranges for mass processing to the document
type.
If there are sufficient number ranges but some of them are already 98%
exhausted, increase the assigned number range. If this is not possible,
assign a new number range.
Make sure that you select number ranges large enough that the document
volume is covered for several years.

Error message extract from SAP system. Copyright SAP SE.