For now, assume the qubits are independent. Each is its own circle; drag any of them. The joint amplitudes are the products, and the number of them doubles with every qubit.
Each circle is one independent qubit, so the joint state is just the product. Two qubits give four amplitudes, three give eight, and the count doubles every time. Notice the joint probabilities factor: P(00) = P_A(0)·P_B(0). This clean picture holds only while the qubits are independent. When they are entangled, it breaks, and no set of circles can capture the state. That is the next section.