If we are talking only about balance and we suppose that the GM is playing NPC fairly (not letting player win), then i always suppose that a group of 4 should always face 4 enemies of the same level. You can tweak the number with a formula like 1 enemy of X lvl is equal to 2 enemy of X-1 lvl. So, a 4 characters group at lvl5 could easly face :
- 4 enemy lvl5
- 1 enemy lvl7
- 2 enemy lvl6
- 1 enemy lvl6 and 2 enemy lvl5
etc...
If players can get goods gears, make NPC get good gears too ! I always allow players to loot used stuff, if they face a archer using Legendary bow, they can get it, etc...
You can always add some skills to your enemies in case they are not strong enough or do an event who help players in case enemies are too strong.
