Narrators will inevitably throw a dangerous challenge at a group of players, and soon discover that either the foes they thought were hardened are a pushover in the face of the players' powers, or that the players are in no way matched to the danger they face, and the narrator may need to think quickly to get them out alive.
Balancing encounters is a tricky situations, and involves being somewhat aware of the overall power level of each group of characters. Narrators should consider using danger points (DP) to assist in this task. For every story point (SP) the narrator hands out, they should record an equal number of DP. The total DP is a rough assessment of the power level of a group of characters. Note that narrators should record the total across all players - that is, if a group of 4 players is each awarded 5SP, the narrator should add 20DP to the current total. In some cases, narrators may wish to record total DP per player, if they expect certain players to not be available in all game sessions.
When constructing an encounter that includes danger, the narrator should consider the current DP of the group, and populate the foes to match. Each entry in the Bestiary includes a DP cost; further, each upgrade available in those entries also includes a DP cost. In addition, for each entry chosen from the bestiary to create the encounter in excess of the number of players, an additional 20DP should be spent (this is to prevent "horde" or "swarm" tactics). The encounter should be built such that the total cost of monsters and upgrades is close to the overall DP of the group.
It does not have to be exact; in fact, it should be expected that the narrator modify the difficulty from there, but the DP value gives a good starting point. The narrator will have to be somewhat aware of how the players are spending their SP. Players may: go deep into combat mechanics (in which case, consider upping the overall difficulty, as these players are prepared); go wide into non-combat mechanics (lower the difficulty here, as these characters will be somewhat handicapped in a fight); hoard their SP (again, lowering the difficulty is a good idea, as the SP haven't even been used yet, so the DP value is a bit inflated).
Keep in mind that while players may choose to focus on combat talents (or not), PCs by design cannot "go deep" in the same way that bestiary entries do. PCs have very limited means of improving their raw attributes, and most tales are designed to "go wide"; coupled with the fact that PCs will be spreading their SP across three tales, even highly synergized tales won't feel as well-oiled as most bestiary entries (which tend to have inflated stats and talents that play to those strengths directly).
When it comes to balancing encounters that do not involve danger, this is often less a case of numbers, and more a case of ability coverage. There is no mechanic to aid in this process - simply being aware of the types of abilities the characters have, and how they might use them to solve certain problems will assist in deciding what sort of challenges to throw at them.