I know that the upper limits for systotic/diastotic blood pressure are 120/80. What is the mathematics behind these numbers? I am curious because I did not find information regarding how the numbers are arrived at. I am not sure if the numbers are based on statistical study or is there a scientific basis for the same.
120/80 are not upper limits on blood pressure - many people have blood pressures far in excess of those numbers. Instead, these are numbers beyond which studies have shown connections to negative health outcomes - heart disease, for example.
There is nothing magical about the numbers 120 or 80 besides their divisibility by 10; a blood pressure of 121/81 is no more risky than 120/80 than 122/82 is over 121/81. Because of this, drawing a hard line isn't possible - it's just a suggestion.
Typically, systolic pressures less than 130-135 aren't given any special consideration beyond normal advice to exercise and eat well, unless there are other conditions/risks. Guidelines from the American Heart Association and the UK's NHS are to consider treatment only for diastolic pressures above 140mmHg. There has been some disagreement about these precise limits, with some recommendations for lower targets, for example here. Some of the disagreement seems to be the types of interventions - the guidelines to treat diastolic hypertension only above 140mmHg are mostly considering medications, whereas the guidelines to treat prehypertension are aiming for improved lifestyle: diet, exercise, etc.