To Civ4's Future Tech, that is. ;) They range in length from 60 to 68 techs. The number of paths of minimum length is two.
So if there are 84 techs in the tech tree, you could at most skip 24 techs on the way.
Some more stats:
Pottery: 2 paths, 2 efficient, 3 techs
Writing: 6 paths, 2...