Sivir has 6 skins (counting PAX, for which even the codes have been disabled): Warrior Princess, Spectacular Sivir (Just wait for the Disney vault to open again! You'll get a chance.), Huntress (Admittedly this is the champions bundle. A little on the expensive side, but better now that flex bundles is a thing.), Bandit, PAX, and Snowstorm.
If you wait until Christmas, they will probably put Snowstorm back in the store if its not already. They usually bring back the previous year's skins each holiday. I think I didn't get her while it was available, they remade it available for a Snow Bundle. I got it that way.
Edit: Welcome to League! If you need anything add me in game, be glad to help!
And to further answer your question. Not randomly. It has to do with what concepts they come up with (the stronger, the better), they have limited resources to publish new skins (can only do so many at once!). But also they have to make money, so unfortunately, popularity can be an influencing factor. But they seem to try to create skins for champs that haven't had one in a while or just don't have many ways for you to celebrate that champion!
As it stands now though, there are like over 350 skins in the game. Maybe 400+. Hard to count.
