Roughly how many stars are expected to collide when Andromeda meets the Milky Way?

Essentially none. Galaxies look crowded because we see them as accumulated light, but the typical separation between stars is millions of times their diameter - the Sun is 1.4 million kilometres across and its nearest neighbour is 40 trillion kilometres away. Two galaxies passing through each other is closer to two swarms of gnats a kilometre apart than to two solid objects.

What does collide is the gas. Interstellar clouds are diffuse but continuous, so they shock, compress, and trigger a burst of star formation - which is why colliding galaxies in Hubble images are lit with hot young blue stars. The gravitational effect on stars is real but indirect: orbits are flung out into long tidal tails, and after a few passes the two discs settle into a single elliptical blob.

The timing is now less certain than the usual four-billion-year figure suggests. Andromeda's sideways motion is extremely hard to measure across 2.5 million light years, and a 2024 analysis including the Large Magellanic Cloud's pull put the probability of an actual merger within ten billion years at only around fifty per cent. It may simply be a near miss.