The core word is "maid", not "mate". The best man is for the groom. The bridesmaid is for the bride. A bride may have a number of bridesmaids but the groom can only have one best man.
The "opposite" or comparable role/title of best man for the primary attendant of the bride is maid-of-honor. The other attendants of the bride are bridesmaids. Other attendants of the groom are groomsmen.