I've not seen any documentation on NumThreadsForParallelMerge, but I expect any table will only be merged on one thread (there's no point otherwise, as they'd be SQL locking issues), so this setting will be the maximum concurrent tables that can be merged.
The optimal value will depend on your hardware configuration. Broadly, the more processor cores (SQL and CRM servers), the higher this can go, but the disk and memory (SQL Server) will also have an impact. Multiple threads can usefully run on the same core
if some threads are waiting for I/O operations. If you have a lot of processor cores (maybe 8 or more per server), and tables larger than the memory available to SQL Server, then you could try increasing this setting.
Microsoft CRM MVP - http://mscrmuk.blogspot.com/ http://www.excitation.co.uk