What is the ground condition within the depth you need to dig? If you have a couple of feet of good topsoil, then you have one situation. However, if there are many rocks, hardpan, etc. you will have an entirely different matter.
I have put in a few dozen posts around here over the past decade, not as a business, but just for boundary fences. The digging conditions here are terrible. I have done a couple of them doing the digging by hand, digging bar and standard two-handled post-hole digger. This is not for the faint of heart!
It is too much work for an old man, and will make an old man from a young man in a hurry!
I did help a neighbor put in about 30 posts a couple of years ago. He rented a digger that worked very well. Some of the posts were located on difficult terrain, so managing the machine was a problem. So, that too is a consideration -- what access? What terrain?
The machine he rented was one that rolled on two wheels (rubber tires, 12"?), with a frame across the wheels. The 6hp Honda engine sat between the wheels. Another frame ran from the rear of the machine to a point in the front. At the point of the frame, the transmission for the auger was connected. It swung on a ball joint. In front of the point was a hoop handle.
The operation could be a one-man operation for the right conditions. The loop handle enabled the operator to hold the auger above the ground, then engage the centrifigal clutch to drive the auger. As the auger dug, the loop handle went further down toward the ground. When the desired depth was achieved, the operator lifted the auger out, stopped the drive, and moved to the next hole.
The man renting the machine said a farmer used it to dig 400 holes in one day -- but good digging conditions, good terrain.
I think an 8" auger was used for the 4X4 posts being used. In this case, the posts were concreted into the holes, allowed to harden, and a wooden fence was built against the posts later. In other words, he was not working with fixed length sections. This is of major concern for your question about "drilling them all at the same time." If you have fixed lengths, then exact placement is very important. Again, the kind of digging conditions is important as to how well you can control the auger.
In this case, he used two bags of QuickCrete for each hole. I did some calculations for him, telling him he should be using three bags. But, he did not want to spend the money, so he used two. The concrete still was nearly 12" below the grade. He filled in the top with soil. He dumped the two bags in the hole dry, leveled, then poured in water, and mixed in the hole. Obviously, we worked with a post level during this operation to insure they were all set true to vertical while the concrete was still pliable.
How are you going to set your posts? If you choose to tamp soil back against the post, this can be a very demanding task if done right. The tamping bar gets very heavy, very quickly. The QuickCrete idea is much easier, although much more costly.
I think the same rental agency now has a Toro Dingo, along with an auger attachment. I can imagine it would do a great job in making the holes.
I don't know if any of this helps, but perhaps give you some considerations you have not thought about before.