This probability measures give us an indication of how uncertain we are about the data. And we use a log2 measure as this represents how many bits we would need to use in order to specify what the class (value of the target attribute) is of a random instance.

Does this mean that if there are more outcomes, for example Nothing, Responded and Sale, then we have to use log3?

I haven't really found a good clear answer to this via Google

Thanks for an informative article.