Count the number of items in my array list

The number of itemIds in your list will be the same as the number of elements in your list:

int itemCount = list.size();

However, if you're looking to count the number of unique itemIds (per @pst) then you should use a set to keep track of them.

Set<String> itemIds = new HashSet<String>();

//...
itemId = p.getItemId();
itemIds.add(itemId);

//... later ...
int uniqueItemIdCount = itemIds.size();

You want to count the number of itemids in your array. Simply use:

int counter=list.size();

Less code increases efficiency. Do not re-invent the wheel...