Personal feeds are private feeds and are limited according to the private feed quotas, depending on your membership level.
Personal feeds also have a quantity limit. Up to 500 items can be saved in one personal feed. If you wish to add another item while you have already reached the limit, you have to delete one of the existing items. Otherwise, you have to add items to other personal feeds.
However, only 50 most recent (according to the item's date) items will be visible in the feed. Other items will be archived. You can manipulate with item's date to make it visible. Or you can remove deprecated items to free room for fresh content. All saved items can be viewed in the personal feed's editor.
Note: personal feeds are still in Beta, some changes to the limits can be applied with time.