|I assume you mean multiple superimposed images (i.e. your camera is on a tripod, you take multiple shots without changing location or framing)?
The easiest way is to keep things to powers of two.
2 images - under-expose each by 1 stop
4 images - under-expose each by 2 stops
8 images - under-expose each by 3 stops
16 images - under-expose each by 4 stops
32 images - under-expose each by 5 stops
64 images - under-expose each by 6 stops
Mind you, you don't have to be spot on. If it was say 55 images, each under-exposed by 6 stops, you'd still have something usable, albeit (marginally) under-exposed. Similarly, if you took 80 images, each under-exposed by 6 stops, you'd still have a usable image that was slightly over-exposed.
If you want to get more precise about it, then rip out a scientific calculator. The number of stops you need to under-expose each image is given by the formula log N / log 2 where N is the total number of images. For example, for 80 images, the result is log 80 / log 2 = 6.32, so you have to underexpose each image by 6.32 stops. In the Windows calculator, you could get this by choosing the scientific view, then entering 80 <log> / 2 <log> =
Enjoy your experimenting! I haven't tried it with a digital camera, but I have done it on film with some interesting results. Make sure your tripod is quite stable (not affected by any breeze, traffic vibrations etc).