This variable has an effect only if you are reading on a key that does not allow duplicates, or if you have added an Identity column to the table. A_MSSQL_ROWCOUNT determines how many rows are returned by a SELECT statement sent to the server.
This variable can be used to speed up the Connector. For example, if only reading one record at a time, and reading from a unique key, you can set A_MSSQL_ROWCOUNT to 1, thus speeding up the processing.
If you know you are going to be reading records ten rows at a time, set A_MSSQL_ROWCOUNT to 10. If you don't have any information about how many rows are going to be requested, set this variable to 0, which is the default.