Your purchase history download list is not limited but free items from PS+ can be removed and have been in the past from users.
Queued Download list
Your active download list is limited to 15 items. No matter the source of the downloads, Either PS3 browser or PSN store, only 15 items are Queable in the list.
PS+ will download 5 Game updates at Each auto download trigger per night. It will do the next 5 Games another night. PS+ downloader will also trigger auto completion of the 15 items on the download list.