How many classes should we have? How does one decide about the size of class interval?

Valid question!!

what all I can say about this is that,once your class size is decided,the number of classes gets decided by itself. Now for deciding what should be you class size,depends on the data you are given.If the data includes values which are close to each other,you should take short class intervals,if the data is widely spread,then take wide class intervals.Try taking intervals of multiple of 5.