SP to return last ID and insert into a column using a trigger
Based on the limited information in your question, I made a few changes to your stored procedure.  If I've misunderstood something, let me know.  This code was not fully tested on my side.

ALTER PROCEDURE [dbo].[SP1_Test] @Opp_Id VARCHAR(24)
    ,@Opp_Number INT
    -- Add the parameters for the stored procedure here
AS
BEGIN
    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    SET NOCOUNT ON;

    DECLARE @NewQno VARCHAR(50)

    --The Qno column will contain the current value, whatever that may be
    INSERT INTO O_QUDF (Client_id,Contact_Number,Type_Id,Code_Id,O_QUDF)
    SELECT @Opp_Id,@Opp_Number,1,0,Qno
    FROM T1 WHERE ID = 1;

    --While you can probably get away with doing simple arithmetic on NVARCHAR columns, 
    --I think it's better to implicitly convert it to a numeric data type to do the arithmetic.  
    --I chose Integer.  
    --Then, you need to convert it back to nvarchar for the upcoming UPDATE
    SELECT @NewQno = Convert(NVARCHAR(50), Convert(INT, Qno) + 1)
    FROM T1 WHERE ID = 1;

    UPDATE T1 SET Qno = @NewQno WHERE ID = 1
END
GO