There is some speculation that most/all star systems (after sufficient time) [self-organize](https://www.sciencedirect.com/science/article/pii/S1384107617301410
) themselves into systems of 7-10 planets. This would be accomplished through Harmonic-Resonance which would cause all matter in an accretion disk to get pushed/pulled into bands at specific intervals from the parent-star.
Evidence of this process exists in our own solar-system, as material between Mars and Jupiter has never coalesced into a planet of its own, but instead is constantly agitated by the bodies around it leaving the matter strewn about in an Asteroid Belt.
Well think about it, we still have trouble determining the location of objects in our own solar system, there is still much debate about the possibility of a large planet on the outskirts of our system with a highly inclined orbit that models of the origin of the solar system suggests exists but for which there is no observational evidence. We can hardly begin to claim to have found all planets in other nearby systems. The system with the most planets I am aware of is the recently famous TRAPPIST-1 system, with 7 confirmed rocky planets, 5 of which are fairly close to Earth sized, of which 3 of those are suspected to reside inside the habitable zone.
Agree with all said before. I would like to ad that the examples that we do have of extrsolar systems with many planets are pretty much all around small stars and the planets have small orbits. This is because it is so much easier/faster to detect planets with a tight orbit. So the statistics that we currently have will not be representative of all systems, but will be skewed towards those systems with tight orbits. We just don't know yet if our current statistics are representative of all/the majority of systems yet. With longer monitoring times (at least three orbits are needed to be considered significant, e.g. examining earth would take three years to have confidence of earth's orbit, and farther out planets would take longer still) we should have a much better idea of this in the next decade.