Nowadays, mounting a base-station (BS) onto an unmanned aerial vehicle (UAV) has become a new dimension for constructing the new generation of wireless communication networks. For example, dynamically constructing an UAV-BS network can provide some instantaneous and emergent communication services when the infrastructure of the cellular network is destroyed owing to some devastating disasters. In this paper, we aim to deploy a scalable and self-organized UAV-assisted device-to-device (D2D) communication network using a minimum number of UAV-BSs (UBSs) under some link quality constraints. Specifically, a sequential UBS deploying algorithm is designed to guarantee the signal quality for the links between the UBSs and ground terminals, and those between UBSs and central controller. Via the simulation results, it is interesting to find that how to deploy a proper number of UBSs rather than constructing a highly connected UBS network is the key to guarantee higher spectrum efficiency for the UBS-assisted D2D networks.