I quite liked the old 1e morale system for monsters. It works quite well for pack/mob type creatures like kobolds. Here is a quick rule you can use to simulate that old rule in 5e.
Once at least one of a groups of monsters number is defeated, the adventurers can compel them to submit with a successful Charisma (Intimidation) check. The DC of the check is 10 + (5 times the number of remaining monsters): for example, if one remains, the DC is 15; if there are two left, the DC is 20, and so on.
The DC should be set appropriately to the level of courage of the creature. Goblins and kobolds may have a DC of 0 while veteran NPCs may have a DC of 15. After a certain point the DM should just roll each round for each creature and not wait for the PCs to make a skill check.