Targets number estimation is an important part to the resolution of unresolved targets. Based on Gerschgorin disks estimation (GDE), we propose a method to estimate unresolved targets number with a single snapshot. The proposed method consists of the virtual image array spatial smoothing (VIASS) and improved Gerschgorin disks estimation (IGDE). A full rank covariance matrix can be obtained while avoiding reduction of effective array aperture by VIASS. Furthermore, this method improves the decision criteria. It's easily applied to radar system since the adaptive detection threshold of IGDE. Several simulation results demonstrate the good performance of this method.