Type: Null is unique in the sense that it can change type to be any type in the game. ... Unfortunately, you can't breed Type: Null so you're stuck to just having one for your playthrough.

Can you breed Silvally?

Silvally is breedable, with the Egg Group "Legendary", so it can only be bred with a Ditto to produce more Type:Null Eggs. If the Type:Null Egg is obtained from the Daycare, then it will count towards your shiny chain.

Can you breed Ditto with type null?

Breeding. It is impossible to breed Type: Null with any pokemon (including Ditto). This is a unique species that does not fit into the gender binary of the Pokemon games and is also in an undiscovered egg group.