The UICollectionViewFlowLayout is a subclass of UICollectionViewLayout. It should therefore have access to the UICollectionView which should know how many items there are.
[self.collectionView numberOfItemsInSection:0];
You may need to iterate over the section to get the total number of items if you have more that one section.
You can get the number of sections similarly:
[self.collectionView numberOfSections];
Hope this helps! :)