When filtering by labels, the list shows up to 20 labels per label type. Because there may be hundreds of labels, this restriction is required to ensure this list does not become too long.
There are a few options for selecting or viewing labels which are not immediately visible in the Labels filter:
If you know the label you need, type in text in the Search field at the top of the Labels filter to reduce the number of labels shown based on your search text.
To see all labels used, go to Management -> Data -> Labels (using the left hand menu). Again, this list has a search field where you can search for labels within a particular label type.