So there have been planets that have been reclassified, planets that, if the theories hold true, have been destroyed or hurled out of the solar system and theoretical planets to explain anomalies in the behaviour of other celestial objects. After all those, we are left with the eight planets we know of today.
There are however unexplained behaviour of different objects in our solar system that suggest there might be more planets to be found.
Planet Nine in Kuiper-belt would explain the perturbations of multiple trans-Neptunian objects. Before that, there was a hypothesis of planet X (as in unknown, not ten) explaining the perturbations of the orbits of Uranus and Neptune. This was later debunked with more precise calculations of these later planet's mass. This search lead to the discovery of Pluto. However Pluto wasn't the looked for planet X as it was too light for the observed anomalies.
The point of origin and clustering of some comets suggested there is a gas giant somewhere far out in the Oort cloud. This planed, named Tyche was however recently ruled out as it should have been visible in the data gathered in recent years.
Going beyond the planets there is also the Nemesis a dwarf star circling our sun far beyond the Oort cloud. The 26 million year mass extinction cycle suggest there is such an object out there that periodically enters the Oort cloud sending increasing amount of comets towards the inner solar system.