When the system "sees" one sub-job, it is a one job submission. As far as the system is concerned there are no sub-jobs.
You could use a Route before the Watermark to check if "Number of Documents in Job Group" is equal to zero. In that case you use a Watermark that prints 1 and otherwise you use a Watermark that prints $FFwfJob.documentGroupCount$.
We create a banner page with information about number of pages and number of envelopes in a print job to the operator.
Number of envelopes equals $FFwfJob.documentGroupCount$ and it works fine from 2 and up.
But if the file only has one file in the group, $FFwfJob.documentGroupCount$ count 0 (zero)
I like it to count '1'