This paper considers distributed medium access control in a wireless multiple access network with an unknown number of users. A multi-packet reception channel is assumed in which all packets should be received successfully if and only if the number of users transmitting in parallel does not exceed a known threshold. We propose a transmission adaptation approach, where, in each time slot, a user estimates the probability of channel availability from the viewpoint of a virtual user and adjusts its transmission probability according to its utility objective and a derived user number estimate. A sufficient condition under which the system should have a unique equilibrium is obtained. Simulation results show that the proposed medium access control algorithm does help users to converge asymptotically to a near optimal transmission probability.