How to declare if yopu don't know the element ammount?

HI,

 

Ive got the following question?

how can i declare an array if you don't know how manny elements the array will finally contain ? 

 

here is my example.  I sed the wcfUser array could contain 1000 elements ( simply because i need to do a test to add 1000 users) but the wcfUser array could at the end just contain for example 15 elements ( you don't know it yet).

 

here is my code;