count of Max(Lookupset) values in SSRS

Hi Friends,

I am using below expression to display list of class codes 

=  Max(IIF(Fields!DayNumber.Value=2,LookupSet(Fields!StaffID.Value & Fields!PeriodNumber.Value 
    , Fields!StaffID.Value & Fields!PeriodNumber.Value 
    , Fi)elds!ClassCode.Value
    , "Dataset1"),"")

Which returns

Class1

Class2 

Now i have to calculate count of class codes but when i use .Length for Max value of Lookupset it is giving error.Please help me how to calculate count for Max(Lookupaset)

Thanks,

Sam