How to implement auto increment column value alphabetical?

Very Nice question.

i) This id can be generated upto how much digit ?like 10,000 or 1,00,000 or more

ii) You should throw little more example randomly start from 101 .specially throw few example from 4 digit and few from 5 digits.

iii)how you are going to implement it.Like will you insert single row or you will insert bulk rows. Implemented SCRIPT can be optimize and change according to requirement once requirement is clear.

iv) Test my script.First thoroughly test it from any number between 101 to 921

Just change the value of @input

DECLARE @Input INT = 127--change here
DECLARE @Output VARCHAR(10) = ''
    ,@TempOutput VARCHAR(10) = ''
DECLARE @Base INT = 100
    ,@z INT = 26
    ,@Rem INT
DECLARE @Tempinput INT = @Input

WHILE (
        @Tempinput > 0
        AND @input > @Base + @z
        )
BEGIN
    SET @TempOutput = ''

    IF (@Tempinput > @Base)
    BEGIN
        SET @Rem = @Tempinput % @Base
    END

    IF (
            (@Rem % @z) = 0
            AND (@Rem / @z) > 1
            )
    BEGIN
        SET @Tempinput = (@Rem / @z) - 1
        SET @Rem = @Rem % @z

        PRINT '23'
    END
    ELSE
    BEGIN
        IF (@Rem > @z)
        BEGIN
            SET @Tempinput = @Rem / @z
            SET @Rem = @Rem % @z

            PRINT @Tempinput
        END
        ELSE
        BEGIN
            SET @Rem = @Tempinput % @z
            SET @Tempinput = @Tempinput / @z
        END
    END

    IF (
            @Rem > 0
            AND @Rem <= @z
            )
        SET @TempOutput = CHAR(@Rem + 64)
    ELSE IF (@Rem = 0)
        SET @TempOutput = 'Z'
    SET @Output = @TempOutput + @Output
END

IF (@input <= @Base + @z)
    SET @Output = CHAR((@input % @Base) + 64)

SELECT cast(@Input AS VARCHAR(10)) + @Output