There is no way to answer that. We don't know how many comets there are, how many there were, or how many actually have hit the Earth in the past. You also have to ask "what is the time scale?".
From looking at impact scars on the Moon, we know that impacts were more common in the distant past (before the evolution of multicellular life) than in the more recent past, but we still are not certain just how many objects there are that could impact us.
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