Frankly, I'd say that the above is going too far with the "drops are a useless unit of measure" and "if you guess wrong, your going to scorch it , just might as well throw it away then"
If you want to get all scientific about it to the point that it's functionally irrelevant, the by all means, do so. Unfortunately, at that point it ceases to be a useful comment and turns into a "look what I know" kind of thing.
Considering the context and keeping extraneous details to a minimum until the relevant information is given goes a longer way toward helping the person than giving every alternative situation available.
I'm going to go out on a limb a bit here and assume that the OP is using his/her bottle that the
juice came in to fill the carts. The opening of the bottle is the same size as the opening on my V4L bottles. The fluid is of a fairly consistent viscosity (yes, some flavors and vg juices are a bit thicker, but for the purpose of this thread they're statistically irrelevant) and most people will drip with a pressure that allows one drop at a time to exit the bottle , so the pressure applied will be effectively even as well.
Considering that the opening, liquid, and pressure are all relatively even, the drop size will be relatively even as well. Since the drop size from one of the V4L bottles is effectively consistent, it very much can be said that number of drops is a good indicator of how much liquid is going into a cart.
Since it was originally stated that the OP was filling Wow and CoolCarts, mentioning "most mega sized cartos..." is irrelevant and simply clouds the issue.
Considering how tightly the cart is stuffed affects how long it takes to become fully saturated, as well as how much fluid can actually be held as well, you could go on all day about how hard it is to tell exactly how much will fill a cart.
I'm not necessarily jumping on the above poster, but I know that when I'm looking for information such as this, I'm looking for a general range from users who are using the same type of stuff that I'm using. I get annoyed with explanations that take the original inquiry off the reservation by trying to toss in every variable. It's not a stretch to assume that someone asking "how many drops" is using the bottles the juice came in, and that the drop size is going to be similar.
How about something like this: "Assuming that you're using the original bottles, most people average X-number of drops, give or take a few. If you use a different filling method, then you'll have to experiment on your own to find your proper range"
If I read a few comments like that one and most of them were in the range I was getting, then I'd have a pretty good idea that I'm in the ballpark. If most of them were saying, say 30-35 and I was barely getting 20 or so, then I'd have an idea that I may need to look at how I'm filling my carts to see if there's something I should look at adjusting.
All of the above aside, trust your eyes. Pop the soft cover off, fill with x-amount of drops, give it a light Taryn spin to make sure the juice has settled in the far end of the cart, and top off and repeat as needed until a light spin still leaves a lightly saturated look on the top of the filler. Count the drops it took to do so. Now you have a good general idea of how many drops it takes you to fill that cart. Done.