It takes around 15-20 transistors to make a simple neuron simulation:

https://www.quora.com/How-many-transistors-can-be-used-to-re...

But: "Synapses are usually separately modeled in transistors (they are not part of the neuron circuits described above) and dramatically add to the transistor count."

"[A neuron] has on average 7000 synaptic connections to other neurons."

So we're not there yet, but it's definitely an improvement!