Adding explanation note on how number of partitions (file splits) are determined.

Author: mcadenaroa

docs/t-sql/statements/copy-into-transact-sql.md

@@ -432,7 +432,7 @@ WITH (
 The COPY command will have better performance depending on your workload. For best loading performance, consider splitting your input into multiple files when loading CSV. This guidance applies to gzip compressed files as well.
 
 ### What is the file splitting guidance for the COPY command loading CSV files?
-Guidance on the number of files is outlined in the table below. Once the recommended number of files are reached, you will have better performance the larger the files. For a simple file splitting experience, refer to the following [documentation](https://techcommunity.microsoft.com/t5/azure-synapse-analytics/how-to-maximize-copy-load-throughput-with-file-splits/ba-p/1314474). 
+Guidance on the number of files is outlined in the table below. Once the recommended number of files are reached, you will have better performance the larger the files. The number of files is determined by number of compute nodes multiplied by 60 - At 6000DWU we have 12 compute nodes for 12*60 = 720 partitions.  For a simple file splitting experience, refer to the following [documentation](https://techcommunity.microsoft.com/t5/azure-synapse-analytics/how-to-maximize-copy-load-throughput-with-file-splits/ba-p/1314474).
 
 | **DWU** | **#Files** |
 | :-----: | :--------: |