Do profit maximizing managers really decide the exact quantity to produce

Do profit maximizing managers really decide the exact quantity to produce (based on the principle that they would continue to produce as long as marginal revenue exceeds marginal cost) or do they make decisions to accept or reject opportunities to produce additional products for a customer order? If they make decisions on whether to accept a new order or not, how do you reconcile that with the marginal revenue equals marginal cost rule”?