The human brain has about 100 billion neurons. That's the number of stars in the milky way galaxy, though you only see about 2,000 in the night sky. Neurons connect to each other in precise and changing ways, but each neuron on average could have about 10,000-100,000 connections to other neurons. That gets us roughly to low quadrillions of connections. I like to think of it as the number of seconds since fish first crawled onto land. This changes over time, so that complexity is dynamic. Then, that's only the neurons, which are less than half the cells in your brain. Blood flow and glial cells also have precise structure. It gets hard to think about how to quantify it, but it is complex.