When we edit the Calculated field so we go Like Analysis --> Edit Calculated Field and there appears "Number of Records" by default I tried to explore it but unable to understand. Why the default number of records?

The number of records is the default field in tableau. When you connect a data set to tableau this is the default field that gets generated along with few other fields. And so, when you are using this number of records in your view it will simply calculate the total number of records in that particular cell. if you want to change this you might change to some other functions like sum(), mean(), max().