I am pretty sure that most of the modern days brush makers could easily keep up with their work load, unless they also make their own knots.
In the first years I was making brushes, I had not much or any competition, maybe Cooncat Bob (and he sent me quite a bit of brush work as he liked more to make his razors) and Elite Razor. I had weeks that I was working every free minute in my shop to keep up with the brush orders. During this time, I also traveled for my real job a lot. So my brush making happened over the weekend. I doubt that a customer had to wait more than two or three weeks for his brush. It is just a different approach - FIFO.
From 2011 on I also made several Limited editions for B&B, sometimes up to 100 pieces. So I could not complain of not being busy.... Next to my "normal" orders, I made these LE's and always delivered before my set time.
In these busy years, my average number of brushes (with out any LE's, or special group buys) was about 8-10 a week. But as I said, I did not make my own knots, but used often hand made knots from Shavemac in Germany. I had to hire a lawn service as I did not have any spare time left for my yard work.
Then all the new makers showed up and they started to do wait lists, drops etc. Personally I doubt they made/make more brushes than I did, so this kind of selling is different approach for me.
Sure, if you make a larger number of brushes for a group buy, LE etc, then you have to estimate the time it takes you to do this job (like procuring the handle material or casting handle blanks, ordering the knots, making sure you have the right sized drill bit etc,) - this, maybe, would be more like a drop.
This is not meant to be a rant or so, and also not to discredit any other makers out there, I just have maybe an old fashioned approach of making a product.