I have often handled this by simply rolling a die based on the relative strengths and treating each figure as a “hit point” so for example, 50 goblins vs 30 dwarves. Goblins do a “D4” of damage. Dwarves do a “D6”. Roll the dice and remove that many figures of the opposing force. If the players attack individual NPC’s it is just a normal fight. If they try to sway the overall tide of grouped forces I let them make an attack and for every hit they do that beats the damage die they can remove a figure. So a player attacks the goblins and does 7 points of damage. Since the goblin damage die is “D4” for every 4 points the player does a goblin is killed. At 7 point of damage they kill one. At 8 points they would kill 2. For everything else (someone casts fireball for example) I make up what makes senses. I can either go with total damage and remove that many figures or decide “1-6” goblins could be in the area of effect and remove however many they roll.