I think the main point to bear in mind here is that if you go back a long way , then composers weren't writing for a particular group of instruments and there wasn't a "standard" orchestra - it was just whatever they could get together.
It's only when we get to around Beethoven's time and later, that parts are written with specific instrumentation in mind and when we get to the Romantic period then what we think of as a "normal" orchestral string section gets set up and written for.
So - Mozart wrote music and "recycled" it for whatever particular combination of instruments were available - he didn't expect that it would be played by large orchestras with 4 - 8 Double Basses and huge numbers of other strings - he was expecting small groups of players,where only occasionally would there even be a single Double Bass....
