Actually as you would observe the number of classes and class interval are related. If we increase the class size the number of classes decreases and vice verse. In case the size of class is taken too large the results are not reliable and in case the number of classes is large the calculations become tedious. It is for this purpose that we recommend that the number of classes should be taken around 15 at this stage. In your question the range is 85-21=64, so we can have different different number of classes but for better reliability and ease of calculation that we should have 6 classes.