In IEEE 802.11p VANET networks the transmission scheduling is distributed and asynchronous. The number of simultaneous transmitters is thus closely related to the CSMA/CA mechanism which limits the spatial reuse of the channel. The capacity is bounded by a constant $C$ whatever the number of nodes and the type of routing schemes. This paper aims to evaluate the spatial reuse of a VANET (using CSMA/CA) and to deduce its maximum capacity. The proposed model is an extension of a classical \textit{packing problem}. We prove formally that the intensity of the maximum number of simultaneous transmitters (the maximal spatial reuse) converges to a constant and propose a simple estimate of this constant. Realistic simulations show that the theoretical capacity offers a very tight bound on the real capacity of the network.