Pivot in Sql

Try below:

ALTER PROC [dbo].[USP_SessionBookPlanningRecommendation] --'0','20,21,22',5
(
@BookID VARCHAR(max),
@StudentID VARCHAR(max),
@NumStudents int
)
AS
BEGIN
    declare @cols as nvarchar(max)='';
    declare @query as nvarchar(max)='';
    declare @tempTable as table (StudentName varchar(max))
    declare @i int = 1

    -- populate temporary table with student names from parameter list
    while (@i <= @NumStudents)
    begin
        insert into @tempTable (StudentName) values ('Student ' + cast(@i as varchar(max)))
        set @i = @i + 1
    end

    select @cols = @cols + QUOTENAME(StudentName) + ',' from (Select distinct StudentName from View_SessionBookPlanningRecommendation where StudentID in (select [value] from string_split(@StudentID,','))) as tmp

    -- add missing student columns to the column list
    select @cols = @cols + ' ' + QUOTENAME(StudentName) + ',' from @tempTable where StudentName not in (select StudentName from (Select distinct StudentName from View_SessionBookPlanningRecommendation where StudentID in (select [value] from string_split(@StudentID,','))) as tmp)

    select @cols = substring(@cols,0,len(@cols))
    set @query = 'select [BookId] [Book ID],BookTitle [Book],[ComboBookName],[AmazonURL],[Core RPL],[Lexile],[Session Plan Status],[Negative List],[ClosingStock], ' + @cols + ' from (select [BookID] ,BookTitle,[ComboBookName],[AmazonURL],[Core RPL],[Lexile],[Session Plan Status],[Negative List],Status,[ClosingStock],StudentName from View_SessionBookPlanningRecommendation where (('''+@BookID+'''=''0'' OR [BookId] IN('+ cast(@BookID as varchar(Max))+')))) x pivot
    ( Max(Status) for StudentName in (' + @cols + ')) piv 
    left join @tempTable t on piv.StudentName = t.StudentName 
    order by BookTitle'

    -- replace null values with "-"
    set @query = 'select [Book ID], [Book], [ComboBookName], [AmazonURL], [Core RPL], [Lexile], [Session Plan Status], [Negative List], [ClosingStock], ' + REPLACE(@cols, ',]', ',ISNULL([' + StudentName + '],''-'')]' ) + ' from (' + @query + ') x'

    execute (@query) 
END

GO

Note that I have added a new parameter, @NumStudents, which specifies the total number of students to display in the result set, including those who do not have a record in the database. The stored procedure creates a temporary table with the specified number of students and left joins it with the pivoted table to include all students in the result set. The REPLACE function is used to replace any null values with "-" in the final SELECT statement.