Is it possible to estimate the ideal number of CPU cores for Android phones?

I'm thinking that with the way we typically use Android phones, run a single app at a time, fewer faster CPU cores are better than many smaller (slower) cores. Does Android really benefit that much from the current octa-core phones? How long before we see 16-core Android phones, and will that actually be useful in a phone? I keep thinking that 4 fast cores, or 2 very fast cores, seem ideal for the kind of things we do with Android phones. Is it possible to determine some ideal number of cores, or at what point it simply does not make sense to add more cores?