Im pretty sure it only goes up to 6. But however some good seeds I have found that have a good amount of rare traits are:
Lil' Mushroom--5 Stars: 087 5638
(This one is cool mainly because of how pretty it is--there will be a few of these ;) ) Prairie Star--Yellow, Orange, and Red--3 Stars: 018 4158
(Pretty) Unknown plant type--2 Stars: 005 1370
(Pretty) Garden Plant--Pink Blossoms with Green Stems--3 Stars: 005 8068
Garden Plant--6 Stars: 009 1347
(This one just looks nice) Garden Plant--Light Blue to Light Purple Blossoms+Green Stems--2 Stars: 009 3653
Hope that helps and brightens up your space!