We don't assume this.
We assume that the Multiverse is as big as it is shown/stated to be. For example, Divinity is stated to have 3 universes at least.
The Godwoke is 2-C. Undertale possibly has an unknown, yet far above 1,000, timeline Multiverse, so
Chara is 2-B.
The only way you get infinite Multiverse Buster status is if your Multiverse has been stated to be infinite.
Even if the Multiverse runs on "each choice/different event leads to a different universe" logic, this still isn't infinite and is just extremely far into 2-B.