If you want to get the count of rows from this linq first save the above expression in a datatable variable using assign activity
dt_output = your above expression
Where dt_output is a variable of type datatable and is defined with default value as New System.Data.Datatable in variable panel itself
If you want to use this datatable further you can use further for any calculation
To just get row count, use a assign activity like this
int_rowcount = dt_output.Rows.Count
Where int_rowcount is a variable of type integer
To know further functionality or calculation on datatable refer this thread
Hope this clarifies
Cheers @Onur_Oz