It's probably unlikely for champions to get multiple Legendary or Ultimate skins since they will not only have a lot of work put into building those skins but they also have to have the Voice Actors available at the time they start working on them.
Most champs will probably only have one Legendary skin. There are of course the older legendary skins on champs like Firefighter Tristana that have nothing more than a new Model and some effect changes, where as her Dragon Trainer skin has ALL of the Legendary features (including a new voice over, animations, death, ect). Others like Cho'gath that have two skins with different voice overs but still also lack a major amount of features, and so on.
So I don't see them really doing multiple legendaries these days since their skin standards have changed drastically.