This depends mainly on the noise level of your camera and the amount of light pollution you need to reduce.
As a general rule:
- Higher noise → More images for stacking.
- More light pollution → More images for stacking.
That said, I have found empirically that (to get a good shot of the galactic center of the Milky Way) stacking about 15 minutes of pictures is more than enough to reduce noise tremendously and therefore be able to apply a lot of light pollution reduction without noise being an impediment to it.
For example, if you set an exposure time of 10 seconds (that’s 6 images per minute) you would have about 90 images for stacking.
Personally, for photographing the Milky Way I usually stack between 75 and 150 images (depending on whether I set a little more or a little less exposure time). But I move in that range of images.