Does this mean that it's highly likely that our solar system had more planets a few billion years ago?
Orbiting bodies are eliminated regularly, either by impacts or by being ejected, the former happening much more often than the latter because the ejection window is smaller than the impact window. And the bigger the body, the less likely it is to happen because to be eliminated it has to hit or do a flyby of something bigger, and the bigger you get the fewer things there are that will do the job. Jupiter, for example, ain't goin' nowhere.
At some point in the past we had a lot more Mercury-to-Mars-sized things floating around than we do now. How long ago the last one got eliminated is anyone's guess, but it was almost certainly a few billion years.
The last one hasn't been eliminated - we still have Mercury and Mars. You probably don't think of it that way because we think of the existing planets as constants, not variables.