So am i correct that you want to use vSAN iSCSI for VMs? Why not run the VMs directly on vSAN? This looks like an extra layer of possible failure.
As for how many hosts to use:
By default, the iSCSI service is disabled, and needs to be manually enabled by the administrator. Every host in the cluster has to participate in the iSCSI target. All hosts in the cluster also need to use same VMkernel adapter. VMware recommends creating a new VMkernel adapter for the vSAN iSCSI traffic. The VMkernel adapter must be the same interface number on all hosts in the cluster.
Have a look at: