In order to have a self-organized multi-agent system exhibit some expected collective behavior, it is necessary to add some special agents with information (called leaders) to intervene the system. Then a fundamental question is: how many such leaders are needed? Naturally the answer depends on the model to be studied. In this paper a typical model proposed by Vicsek et al. is used for answering the question. By estimating the characteristics concerning the initial states of all agents and analyzing the system dynamics, we provide lower bounds on the ratio of leaders needed to guarantee the expected consensus.