C :: How To Make Array Of Structures For Basic Contact List In A Phone

So for class I have to make an array of structures for a basic contact list in a phone.

I understand the bones of the program and how to go about doing most of it but as far as arrays of structures go I am blind.
Code:

struct phone
{
char FirstName[16];
char LastName[16];
int Number[11];
};
struct phone numbers[friends]; //friends is a variable assigned by the user What I am a bit confused about is say the user enters 30 as how many friends they have. How would I assign a value to the 3rd struct for LastName?