Get Json field value from Json
Assuming you are on SQL Server 2016 or greater, you can use the JSON_VALUE function to parse out an individual value:

SELECT  JSON_VALUE(@JSON,'$.ErpOrderResponses[0].OrderNumber');


If you are going to be processing more than one row, I recommend you use OPENJSON for better performance:

SELECT 
    * 
FROM 
    OPENJSON(@JSON,'$.ErpOrderResponses')  
    WITH (OrderSubmitStatus varchar(20),OrderNumber bigint);


Documentation: https://docs.microsoft.com/en-us/sql/t-sql/functions/openjson-transact-sql?view=sql-server-2017