Hi @Tharindu Illangasinghe ,
If you select RMA lines and click on Create Credit Invoice button, separate credit invoices will be created for each RMA line. Instead, if you create the credit invoice from RMA header (without selecting any RMA line), one credit invoice will be created for the whole RMA, including all RMA lines. Thanks.
Hi @Ravinatha Ariyarathna,
Nice to hear from you and thank you for the response 
Is it possible to select a set of lines (not all) in the RMA and create a single Credit Invoice ?
Or is there a Company/Customer setting to achieve that ? Because users seemed to recall that they were kind of able to do that.
HI @Tharindu Illangasinghe , thanks, 
As I know, there is no setting in Site / Company level regarding this.
I believe what you can do is select the set of RMA lines need to be invoiced and approve them for credit (then other RMA lines are not approved for credit). Next you may create the credit invoice from the RMA header. This will create one credit invoice only for the credit approved lines.
The remaining RMA lines could be approved for credit later and create credit invoice(s) for them as needed.
Note: It is also possible to revoke the credit approval of an RMA line if needed.
Hope this helps you.
Best regards!