1 or 2 S/4HANA instance architecture?

Hi, we are exploring a 1 or 2 instance strategy for a S/4HANA private cloud architecture. 

We have an SAP S/4HANA Private Cloud – Utilities (Meter to Cash) instance that we use for customer billing.  We also have a legacy ECC 6/HANA instance supporting Finance and Supply Chain processes and are planning on upgrading this instance (conversion) to S/4HANA private cloud in the future.  

Is it better to have an architecture of 1 or 2 S/4HANA private cloud instances in this case?  

Thanks.