Its my pleasure, anyways why dnt  you get the highest id number in this case or pass the highest id number as parameter...

alter function NextCustomerNumber

returns char(7)
as
begin

set @id = select max(ID) from table_name ;--to get the highest id number from the table
return 'R' + right('000000' + convert(varchar(10), @id), 6)
end;

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OR

begin

set @id = select max(ID) from table_name ;--

select NextCustomerNumber(@id) ....

END;