I see what you’re saying. Before Doc disappears in the thunderstorm to 1885, there is not a DeLorean in the mine. But immediately after, when Marty gets the letter, there now is one in the mine, but there aren’t any other DeLoreans in 1955 at that moment.
However, that now means, retroactively, the one in the mine now always was there, and anyone living through that newly merged 1955 would be sharing Hill Valley with four DeLoreans.
If I could go back in time, I’d have stuck in a DeLorean driving past in the background somewhere in the third or fourth movie, just to screw with people like us down the road.