A pair of engines will have a direct connect storage bay and can have multiple daisy chain storage bays depending on the engine count and whether you want to limit the array to 4 engines or be able to add more engines in the future. If you have four engines you can implement what are referred to as extended drive loops but that will limit the array to 4 engines only and you won't ever be able to add more engines once you start down that path.  With your current configuration of 4 engines and 2 storage bays you have a direct attach bay for Engines 4 and 5 on the left (looking at the front of the array), and a direct attach bay for engines 3 and 6 on the right. You can add one daisy chain storage bay to each of those direct attach bays before having to make the decision to go with extended loop drive bays or add more engines and drive bays.  As Quincy said your local SE should be able to help you make the decision on what is best for your workload in the long term but at a minimum you can still add two more storage bays with the current setup.
We have a VMAX 20k with 4 Engines with Open Systems and IBM mainframe volumes.  We have 1 system bay and 2 drive bays.  Soon we will need to add another drive bay.  At what point will we need to add new Engines?  We seem to have good performance.  Our important Storage Groups have response time less than 2 ms. The front end and back end are usually around 18% busy.  The array generally does about 9000 IOPS and 400 MBs (of course we get spikes at night during batch and backups).  I was thinking we could keep from adding engines until we get to 4 or 5 drive bays - probably in 2017.
VMAX 20k Number of Engines