A steel rivet 5.0 mm across joins two plates…

Step 1. Area. $\pi(2.5 \times 10^{-3}\ \text{m})^2 = 1.96 \times 10^{-5}\ \text{m}^2$.

Step 2. One rivet. $3.2 \times 10^{8}\ \text{Pa} \times 1.96 \times 10^{-5}\ \text{m}^2 = 6283\ \text{N}$.

Step 3. How many. $\dfrac{4.0 \times 10^{4}\ \text{N}}{6283\ \text{N}} = 6.4$, so $7$ rivets.