For example start from B2 to the last row in B column.
Sheets("FC").Range("b2").End(xlDown).Row
or
Example: Read all row in the column and add to combo box.
For line= 2 To wb1.Sheets(1).Range("b2").End(xlDown).Row
ThisWorkbook.Sheets("Sheet1").ComboBox1.AddItem wb1.Sheets(1).Cells(satir, 2).Value
Next