Unless they were a double-planet, similar to Pluto/Charon. (Those orbit a common center not inside either body.) To my knowledge, they are unique, in that, among known bodies, though it would not be surprising if there were a few tiny double asteroids.
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How likely is that there are multiple smaller planets instead of one 9th, on the same side working as offset?
They’d have different orbits with different orbital periods so wouldn’t stay “on the same side” over millions of years.
Unless they were a double-planet, similar to Pluto/Charon. (Those orbit a common center not inside either body.) To my knowledge, they are unique, in that, among known bodies, though it would not be surprising if there were a few tiny double asteroids.
Since we are talking about ~350AU (Pluto is at ~40AU), that may very well be the case.
An easy approximation:
https://www.sjsu.edu/faculty/watkins/bode.htm
Given how far out they are, how far off the solar system axis, and how small, it's definitely possible that there are multiple objects.