Demand for Operations Management textbook $($by Dr$.$ Stevenson, sold at Amazon$)$ is $12,000$ units per year. Amazon incurs a fixed ordering cost of $$400$ each time an order is placed from the publisher. Each textbook cost Amazon $$50$ and it is estimated that Amazon has an annual holding cost of $20\%$ for each book it holds. Compute the number of textbooks an Amazon warehouse manager should order in each replenishment lot.