borje, you asked the unanswered question!
How many tracks depends on the density of each
track, how many plug ins are used, the complexity
of each plug-in, etc. The echo reporter advice
from jrlemonz is great advice, indeed. Take
the number of tracks it gives and figure several
tracks less if you do fancy shit.
Some of my stuff is twenty minutes long, kinda
cuts done the trax! Skol, David