Not an airliner per se, but FAR Part 25 cert, so it might be a useful guide.
My company have done two "new design" certifications in the last ten years or so - Global Express and Challenger 300 (nee Continental)
The GX test programme had 4 test aircraft at various stages, and probably about 1000 test flights over an approx 2 year period.
The Challenger 300 had 5 aircraft involved, and a similar duration and number of flights.
There's probably some "official" numbers around - it's the kind of thing they like to stick on press releases when we get type approval or first delivery. If you need better than a WAG (which is what the above is) it might be possible to find those.