Ultra-WideBand (UWB) localization has shown promising prospects in both academia and industry. However, accurate UWB localization for a large number of tags (i.e., targets) is still an open problem. Existing works usually require tedious time synchronization and labor-intensive calibrations. We present VULoc, an accurate UWB localization system with high scalability for an unlimited number of targets, which significantly reduces synchronization and calibration overhead. The key idea of VULoc is an accurate localization method based on passive reception without time synchronization. Specifically, we propose a novel virtual-Two Way Ranging (V-TWR) method to enable accurate localization for an unlimited number of tags. We theoretically analyze the performance of our method and show its superiority. We leverage redundant ranging packets among anchors with known positions to infer a range mapping for auto-calibration, which eliminates the ranging bias arising from the hardware and multipath issues. We finally design an anchor scheduling algorithm, which estimates reception quality for adaptive anchor selection to minimize the influence of NLOS. We implement VULoc with DW1000 chips and extensively evaluate its performance in various environments. The results show that VULoc can achieve accurate localization with a median error of 10.5 cm and 90% error of 15.7 cm, reducing the error of ATLAS (an open-source TDOA-based UWB localization system) by 57.6% while supporting countless targets with no synchronization and low calibration overhead.