Super-quick guidelines for a "nice" encounter:
1) if the foes have a greater Defense than the PCs (on average), have one less foe than PCs.
2) if the foes have about the same Defense as the PCs (on average), have the same number of foes as PCs.
3) if the foes have a lower Defense than the PCs (on average), have one more foe than PCs.
This ain't D&D, so the expectation is really that as the Games Master you need to learn the system, as well as the strengths and weaknesses of your players' characters
, and make up encounters suitable for your personal group.
Much like, well, almost every other game on the market except D&D, really!