The question of warm-up sets has been contrary for a while in the fitness world...
How many sets do you do? How many reps do you do? How much weight do you use?
Whats too much? Whats too little?
Do you even need to warm up?
To be honest, I try and do the least amount of warm-up sets and reps as I possibly can & you should too. Me personally, I usually perform 2-3 warm up sets before heading into my actual work sets; though, this is primarily dependent upon how warmed up I am already feeling.
If it's the first exercise of the session, you're probably going to need some type of warm up to help get you going.
If it's after your first exercise of the session, you might not need to warm up as much, especially if you're training the same muscle group as you did previously. If you're introducing a new exercise to a new muscle group in the same workout, it would be wise to do some type of warm up beforehand before loading up the weight.
Ideally, less warm up sets and reps are better, if you don't need them.
Many people make a mistake of doing too many reps at too close of a weight to their working set. This isn't ideal because it fatigues you before you even reach your main sets for the workout, which are the ones MOST IMPORTANT for targeting muscle strength and growth!
One really popular strategy is the 5, 3, 1 method. This is a really good strategy because the 5 reps for the first warm up set are at a low enough weight where it won't fatigue the muscles or nervous system, yet there are still enough reps to actually warm up the muscles.
Doing 1 rep on the final warm up set is pivotal because you want to be as fresh as you can before your first work set as this is likely going to be the set you will try to PR on. 1 rep is sufficient here as it your body and gets it ready to handle the heavy load of your first workout set, all while, not actually fatiguing you beforehand.
In fact, if you perform this rep explosively, you will fire up your nervous system without fatiguing it.
This strategy will help recruit more muscle fibers during the first work set and will actually aid in us being stronger and getting the most out of our first work set afterwards. Our nervous system is essentially "primed" going into our first work set having us performed the last workout set explosively.
Personally, at most I'll do 2 reps on my last warm up sets, even if my first work set will consist of 8 or more reps. The key here is that we want to prepare our body to handle the load of the weight we will be using, and prime the nervous system without putting too much strain on it.
That's the purpose of warm up sets... finding the happy dose that allows us to warm up, prepare our body for the load, and fire up the nervous system that way we are strong for our work sets.
Too much, and we fatigue ourselves a little bit before going into our actual work sets. This leads to us not consistently hitting PR's or being weaker than we should for our most important sets.
Too little, and we risk being weak during our work sets as well but for different reasons. Ever feel week during your first work set, and then the next set is much easier? same science described above.
In essence, the less warm up sets you can do to accomplish the above is the best. The goal is to be as fresh as you can possibly can for your work sets, while still actually being warm-up and ready to attack and explode through that first set!