Well, swarms of various vermin for one. You can look at how they work and potentially apply it as a sort of template to larger creatures like goblins.
There was an Unearthed Arcana for mass combat, but that's more for army vs. army.
As long as all the enemies are similarly equipped, 5e makes it pretty trivial to slay through hordes of goblins without a ton of bookkeeping. Or just abstract it, like how a skill roll lets you accomplish a series of complicated gestures to pick a lock or investigate a murder scene. For example, instead of rolling attack to hit, you roll attack to see how many trivial foes you kill or turn back. Then the mob rolls to swarm you with damage.
Are there any guidelines or rules for making mobs. Having so many low level monsters that they just bunch into one entity.
Well, swarms of various vermin for one. You can look at how they work and potentially apply it as a sort of template to larger creatures like goblins.
There was an Unearthed Arcana for mass combat, but that's more for army vs. army.
As long as all the enemies are similarly equipped, 5e makes it pretty trivial to slay through hordes of goblins without a ton of bookkeeping. Or just abstract it, like how a skill roll lets you accomplish a series of complicated gestures to pick a lock or investigate a murder scene. For example, instead of rolling attack to hit, you roll attack to see how many trivial foes you kill or turn back. Then the mob rolls to swarm you with damage.
Even a blind squirrel finds a nut once in awhile.
This section of the DMG contains a simple rule to handle a mob of monsters.