Hello Santosh,
You can create only one Service Account but you have to create separate servicePrincipalName (SPN) for every instance following this example SAP/SAPService<SID> where the <SID> will be the instance.
The best is to use the sncwizard. See the details here: Using the Single Sign-On Wizard to Configure SNC and SPNego - What Is Secure Login? - SAP Library
Regards,
Donka Dimitrova