I believe it depends on the approach how you and your customer would like to manage the gateways.
For example, if you are a MSSP that will manage multiple customers gateways and your service will not allow customer to access the Spark Management Poral (SMP) cloud features, it is possible to have only one SMP domain and manage different customers gateways in the same domain/account.
On the other hand, if you would like your customers to be able to access the SMP cloud features, but you would not want them to able to see your other customers gateways, the choice would be to create an individual Infinity Portal account for the customer.
In this case, either you create an Infinity Portal account for customer and add their email address for admin access, or vice versa your customer can create the Infinity Portal account and add your email address if they need your support.