PowerPivot Tables

I have a table called 'Car', in it i have a column called PolicyLink. the PolicyLink column contains numbers such as 445577, 445578, 445579, 445579, 445579. I am trying to associate how many cars are on a policy and what number that car will be given on the policy so I have a PolicyLink number and if it repeated more than once then the numbers will go in sequencial fasion until there is a new policy.



PolicyLink --------------------- Car Number

445577 --------------------- 1

445578 -------------------- 1

445579 --------------------- 1

445579 --------------------- 2

445579 --------------------- 3

here i have 3 policies so i have the first policy having one car, second having one car, and the third having three cars and so on and so forth. I am looking for a formula to insert into a new column in PowerPivot called 'car number'. the number under PolicyLink is PolicyLink and the -- just represent a change in column, so there is a column called 'car number' with single digits in it. Any help is appretiated