Well it depends on many factors. If you are working just one or related muscle groups you won't need all that many warmups after the first exercise. Say it's just a back workout and you are starting with lat pulldowns, you might do 50 80 100 140 or whatever is your top set(s). If you do rows next you don't need to start from the bottom, 1-2 sets might be enough to get to your top weight. Also different movements require shorter/longer warmups. Think curls vs dead lifts.
If you are doing chest then back, warmups will have to be fairly complete for both in my opinion.