I'm not sure I completely understand your question. A Solaris system running ZFS can provide iSCSI storage to other hosts backed by ZFS volumes ("zvols"). FreeBSD can do this too, although the iSCSI implementation is neither as nice nor as integrated with ZFS.
An iSCSI LUN can be shared by multiple writers -- provided that they're running the appropriate cluster-aware filesystem to support the necessary coordination. Examples of cluster filesystems are VMFS (used by VMware), GFS, GPFS, and so forth.
An iSCSI LUN can easily be shared by multiple hosts if only one is using it at a given time (for example, in a failover environment where one host will take over if the primary host fails).
The limit on how many hosts you can serve is not available NIC ports...it is (a) your available bandwidth and (b) your IO requirements. If you're not doing intensive io, you can happily support many machines over a single gigabit NIC.