We've just seen the one, but we've only had the capability to detect something like that for ~5 years. So in the absence of further data, it's reasonable to conclude that `Oumuamua-like objects pass through the solar system once every 10 years.
Of course smaller and larger interstellar objects will also pass through the solar system. Objects half the size -- which presently we'd be quite lucky to detect -- should pass through with ~5 times the frequency, assuming a size/frequency distribution similar to asteroids. Objects with a quarter the size (currently almost impossible to detect) should be ~25 times as frequent, etc. There is presumably a lower bound to this: at some minimum size, interstellar objects will be repelled by light pressure and the solar wind. I have no idea what this size might be, but for argument's sake let's say that is 1/8th the size of `Oumuamua.
So for any given year, the probability of interstellar objects passing through the solar system, should be as follows, (where `Oumuamua is size=1):
Size | Annual probability
-------+-------------
0.125 | 12.5
0.25 | 2.5
0.5 | 0.5
1 | 0.1
2 | 0.02
4 | 0.004
8 | 0.0008
...there's not really an upper bound, but I can make my point without one. My point is, in any given year, based on current observations, there should be roughly 16 interstellar objects passing through the inner solar system.
Multiplying that by the ~4 billion years that the Earth has been around to run into, and you get a very large number. It seems certain that some small fraction of that large number would have run into Earth. So based on this back-of-the-envelope analysis, either the actual probabilities are MUCH lower -- as in thousands of times lower -- and we were unbelievably lucky to see `Oumuamua when we did, or Carl Sagan was wrong.