How they determine how much money you get from a Will?

First, the will determines how much you get.  You are entitled to a copy.  Make sure you are getting the right amount.  Second, sometimes a will provides for certain payments before others, like debts.  If there is not enough money to make all payments, some will be reduced.  That is why it is often not advisable to put specific dollar numbers in a will, but give percentages.