A different approach than the mainstream view of how many followed Satan.
Revelations 12:4 "And his tail drew the third part of the stars of heaven, and did cast them to the earth:..."
D&C 29:36 "... — for, behold, the devil was before Adam, for he rebelled against me, saying, Give me thine honor, which is my power; and also a third part of the hosts of heaven turned he away from me because of their agency;"
Before diving into this topic I need to give credit where credit is due, what I'm going to give an interpretation of is something that my father assembled and thought upon before drawing this conclusion; I just felt like putting it on the internet.
The common understanding of the result of the War in Heaven is that 1/3 of the Hosts of Heaven, or 33%, fell away with Satan and were cast down to the earth, to dwell here and wreak havoc among men, and to do all they can to entice us to sin and stray from the Plan. Now, I've quoted the exact scripture from Revelations and you'll notice that it states "... the third part ...", which would indicate there are two other parts. I can take a pie and slice it into 3 pieces and end up with 3 parts, they don't remotely have to be even or equal to be classified as three parts. One part could in fact be more than half of the pie, another a mere sliver, and the third whatever is left. Our first proposal then, is that not all parts are created equal.
The other two parts
So the question that would then follow is; who/what are the other 2 parts then? Let's dive into the scriptures and see. Doctrine and Covenants section 76 is one of the greatest revelations on heaven ever recorded. In this vision, the prophet Joseph Smith saw the War in Heaven, and wrote the following: (verses 25-27)
"And this we saw also, and bear record, that an angel of God who was in authority in the presence of God, who rebelled against the Only Begotten Son whom the Father loved and who was in the bosom of the Father, was thrust down from the presence of God and the Son. And was called Perdition, for the heavens wept over him—he was Lucifer, a son of the morning. And we beheld, and lo, he is fallen! is fallen, even a son of the morning!"
From this we learn that Lucifer was a son of the morning, and had fallen. There must be something about being a son of the morning, moreso than just being a spirit in the pre-existence. Continuing on, let's go to Job and read chapter 38: 4-8:
"Where wast thou when I laid the foundations of the earth? declare, if thou hast understanding. Who hath laid the measures thereof, if thou knowest? or who hath stretched the line upon it? Whereupon are the foundations thereof fastened? or who laid the corner stone thereof; When the morning stars sang together, and all the sons of God shouted for joy?
Can you see where I'm going with that verse? There is a distinction between "the morning stars" and "all the sons of God". Two parts, two groups, two congregations of spirits. We learn more about what really separated these two parts; it wasn't God choosing favorites. In Alma 13 we read: (verses 3-5)
"... —being called and prepared from the foundation of the world according to the forknowledge of God, on account of their exceeding faith and good works, ... And thus they have been called to this holy calling on account of their faith, while others would reject the Spirit of God on account of the blindness of their hearts and blindness of their minds, while, if it had not been for this they might have had as great privilege as their brethren. Or in fine, in the first place they were on the same standing with their brethren; ..."
From this we learn that from the foundation of the world the Gospel of Jesus Christ was being taught to the spirits in heaven, some rejected it, and some had faith to accept it and were called to the Priesthood; this creating two groups, two parts of heaven.
I would then propose that after this exerise of faith we have all the sons of God, and then we have the morning stars, or they who exercised faith. Satan was one of the spirit children that exercised faith, that became a morning star, he became a son of the morning, and at sometime in that grand council of the 'great and noble ones' he proposed his plan for saving men, was rejected, and he fell from that grand council and drew away a new part, a third part of the hosts of heaven thus being created.
There is no way to deduce the number of spirits that would have followed Satan and lost their first estate, lost forever the chance to obtain a body and progress from Glory to Glory until reaching exhaltation. What we do know, however, is that we will always have power over him if we exercise enough faith in the Savior; Satan is bound by the rules of the Gospel just as much as you or I, and when faith in the Savior is exercised, we will always have power over Satan.
Therefore in conclusion, I would suggest that it wasn't 33% of the hosts of heaven that were cast out with Lucifer, but probably a much smaller number in comparison to the spirit children (you and I) that decided to accept the gospel and follow the Savior; or to at least stick with the majority, come to earth and see what it was all about. By realizing just that simple fact, that in the pre-existance you had faith enough to believe in Jesus Christ and come to earth to be tested, you can begin to see the nature of who you really are, that you are not a small person in the grand scheme of things, but rather someone that has already proven yourself to God that you're willing to do all that's necessary to obtain exhaltation. If we could only part the veil and see ourselves in the pre-existant state, I would daresay that most of us, if not all, would be amazed at ourselves and what we did before coming to earth. Thankfully, through prayer and faith we can, through the Spirit, gain a testimony of our own importance; of what it really means to be a Child of God.
A great illustration of Lucifer being cast out of heaven to dwell on the earth and wreak havoc on its' inhabitants.