As we've seen in previous installments, the Hazzan at Arbit may proceed to Kaddish after the Amida as long as at least six men (including him) have completed the Amida. During the other prayers, when the Amida is repeated, the Hazzan should wait for at least nine people (including him) to complete silent Amida before beginning the repetition (unless there is an urgent time constraint).
Ideally, the Hazzan should wait for everyone in the synagogue to complete the Amida, in order to give them all the opportunity to fulfill the Misva of responding to Kaddish or to the repetition. Although the Hazzan is technically permitted to begin as long as the minimum required number of men are responding, nevertheless, he should, ideally, wait for everyone in attendance to finish.
In practice, though, the interest in giving everyone the opportunity to respond must be weighed against the concern of "Torah Sibur" – causing undue inconvenience to the congregation. Halacha accords significant weight to this consideration, to ensuring that the prayer service does not become too difficult an imposition. Waiting for everyone to finish the silent Amida – especially in a large congregation – can often cause an unreasonable delay. Moreover, an individual who chooses to recite the Amida very slowly should not be given the power to delay the entire congregation by making the Hazzan wait for him to finish.
Thus, common sense is needed to carefully balance these two conflicting interests – the interest in giving everyone the opportunity to respond to Kaddish or the Hazzan's repetition of the Amida, and the interest in not inconveniencing those who have admirably taken time from their busy schedules to pray with a Minyan. The Hazzan should try to wait for as many people to finish as reasonably possible, without causing a delay that turns the prayer service into a burden.