Currently you can only assign one "NAS" to a single group and pool.  That NAS unit could consist of one or two head units.  If you have two, then it forms a single cluster and they both will access the same pool.   So, yes currently in order to serve our 1PB you would need two FS76x0's and two EQL groups with 509TB each.   As newer, larger drives are available that will increase.  I would check with your sales/reseller to get a presentation of the product roadmap.
Hi, Previously I have been informed here that :- "A "single" FS76x0 comes as a 2 node (controller) unit which can serve up to  509TB. If you add a 2nd FS76x0 and form a "cluster" all the number are doubled which mean 1000TB."   When speaking to Dell to attempt to design and buy a NAS solution to complement/integrate our EQL PS storage I have now had some contradictory information regarding what you get by adding a second FS7610 and forming a cluster within a single group/pool, namely that the capacity is the same whether there are 1 or 2 FS7610 nodes.  Note the last sentence. Either way it doesn't seem I can get 1PB without having at least 2 EQL groups on the same infrastructure, each with 1 or more FS7610 and up to 8 enclosures. However if what I have previously been told here is true I can at least postpone that for some time as I will be able to go 4-5 years before I reach the limit of one group/pool/nas.  Scalability The FS76x0 and EqualLogic SAN provide a file and block solution that scales easily and efficiently as customer needs grow over time. The flexible architecture of Fluid File System with PS Series arrays enables expansion of storage capacity and performance capabilities within a single namespace. The FS appliance can address up to a maximum of 509TB usable capacity available from a single EqualLogic storage pool. An EqualLogic pool can consist of 1 to 8 PS series arrays (members). Notes:   One EqualLogic Group can have only one NAS cluster. A NAS cluster consists of one or two FS appliances providing file services. ·One EqualLogic Group can have one or two FS76x0 NAS appliances operating together in one NAS cluster. ·A NAS cluster consisting of one or two FS76x0 appliances in an EqualLogic Group can access storage for file from a single EqualLogic pool only. One cannot assign one FS76x0 appliance to a pool and another FS76x0 appliance to another pool. · Maximum file capacity that a NAS cluster (and hence one EqualLogic group) can export is 509TB usable (all from a single EqualLogic pool).      
FS7610 and EQL group/pool limits