Step 1 :Substitute \(t=12\) into the function \(N(t)\) to get \(N(12)=\frac{12,000}{1+999 e^{-12}}\).
Step 2 :Calculate the value of the exponent: \(e^{-12} \approx 6.14421235332821 \times 10^{-6}\).
Step 3 :Multiply this value by 999: \(999 \times 6.14421235332821 \times 10^{-6} \approx 0.0061380679409946\).
Step 4 :Add 1 to this result: \(1 + 0.0061380679409946 \approx 1.0061380679409946\).
Step 5 :Divide 12,000 by this result: \(\frac{12,000}{1.0061380679409946} \approx 11922.22\).
Step 6 :Round to the nearest person to get the final answer: \(\boxed{11922}\) people have caught the flu after 12 weeks.