You're going to need to choose between iSCSI and NFS, as they are different protocols for acheiving basically the same thing -- connecting to storage. NFS is file-based, where the filesystem lives on your NetApp and ESXi works with those files. iSCSI is block-based, where the NetApp presents a LUN to the ESXi hosts, and a VMFS filesystem is created on the LUN. Both are IP-based, and you should discuss implementation with a networking engineer, especially to separate storage traffic from your other network traffic with VLANs, etc.
As for how many and what size volumes to create, it depends on how you will use them. It would be easiest to just create one large volume, especially since all volumes on an aggregate (NetApp) will share i/o from the same disk spindles. But since snapshots are volume-based, your backup policies may call for more than one volume. Also consider redundancy -- if you have only one volume and you fill it up, all VMs on it could be affected.
I recommend getting a book on vsphere to learn the basics.
