The principal culprit was named neuron, and for a long time the answer looked similar to 100 billion. Recent studies warn about a possible difference of about several billion less, so it's still in a cloudy state. However, the number seems rather high and very expensive in terms of energy consumption.
We should remind that the human organism and almost every other works thanks to the synapses, due to electrical signals. For this reason, the analogy with technologies that try to emulate reasoning and movement with electricity is, like it or not, inevitable.
Other species have different numbers of neurons, although it's also obvious that not every neuron performs exactly equal, so why not a neuron could outperform ten? The difficulty maybe resides not in to count but in trying to discover differences of this kind.
With so many neurons, or the ability to create more the person could be optimistic about being intelligent. Nevertheless as the brain has multiple functions, the information is scarce about what they do for a living. Employed for memory, they can't be really helpful in some circumstances.
But automatically the involved magnitude of neurons reminds how complex looks the very common interest of simplicity. Even when someone tries it, there are too many factors to create scales or for trying to measure what's possible. To say black or white, or a numeric IQ sequence is easier. Between the most important aspects of the human brain appear the capability of learning, imagination and progress and without effort, the differences start to be very big because the brain should optimize for storing valuable energy.