There's a maximum to the number of al_engine processes running simultaneously, determined by the value of MAX_NO_OF_PROCESSES in dsconfig.txt. Default value is 8. You can increase that parameter, but I doubt it will make any difference for performance, IO being the bottleneck.
The massive amount of IO's is partly due to the huge redo log content generated in SQL Server. You cannot disable logging completely like you can in other database management systems, which helps a lot 
. You can only minimize it by switching your database to the simple recovery model.