In this paper, we systematically study the audio-visual speech separation task in a multi-speaker scenario. Given the facial information of each speaker, the goal of this task is to separate the corresponding speech from the mixed speech. The existing works are designed for speech separation in a controlled setting with a fixed number of speakers (mostly 2 or 3 speakers), which seems to be impractical for real applications. As a result, we try to utilize a single model to separate the voices with a variable number of speakers. Based on the observation, there are two prominent issues for multi-speaker separation: 1) There are some noisy voice pieces belonging to other speakers in the separation results; 2) Part of the target speech is missing after separation. Accordingly, we propose \textbf{BFRNet}, including a {\bf B}asic audio-visual speech separator and a Filter-Recovery Network (\textbf{FRNet}). FRNet can refine the coarse audio separated by basic audio-visual speech separator. To have fair comparisons, we build a comprehensive benchmark for multi-speaker audio-visual speech separation to verify the performance of various methods. Experimental results show that our method is able to achieve the state-of-the-art performance. Furthermore, we also find that FRNet can boost the performance of other off-the-shelf speech separators, which exhibits its ability of generalization.