The current systematic review evaluated influence of implant number on peri-implant marginal bone loss (MBL) and number of implant failures in mandibular implant overdentures (MIODs). Based on the findings of this meta-analysis, no recommendations could be made with regards of implant number for completely edentulous patients with MIODs. Though single implant was found to be better in terms of MBL and number of implant failures, this result should be interpreted with cautious due to limited number of analysed studies with different loading protocols and limited follow-up period.