There is no limitation or restriction as such. You can put the entire array in the same subnet (including the management port), if your envirnoment is setup like that. It is a best practice recommendation to keep iSCSI traffic in a separate subnet for optimum performance. Also, in a multiple router/switch network, it gives fault tolerance if there are multiple subnets/networks.
Is there any reason why I cant just configure all four ports on a VNXe 3150 to be on the same subnet?
VNX 3150 Multiple Subnets or Not?