You sound like you may be considering single flavor testing (since you mentioned 10ml). If that’s correct, you’ll want to at least have 30-40 of those, which will accommodate testing 10-15 flavors at a time.
Regardless of single flavor testing though, I believe you’ll want to double the size of your 15-30ml supply. As if you intended on steeping, consider half of those bottles “gone” as they will be simply sitting in a drawer somewhere, and won’t be available for recipes that cure quickly (surprising how many recipes have creams, or Custards, or…that need to set up for 3-4 weeks or more).
Depends on the bottle, but absolutely great tip, which brings up another point for the new mixer to be on the lookout for!
All bottles are not created equally!!
That is to say, there’s a LOT of crap peddled (sold) as being a 30ml (same for the 15ml), that are not actually what they say, as they don’t include the room for off-gassing the alcohol (the upper portion just before the neck) that typically occurs during the steeping process.
My recommendation would be VM’s, but a step further.
Each time you get a new batch of bottles, take one of each size, and verify that it holds what they said it does. (Use PG as that’s easier to work with.)
IE: 15ml bottle. if you measure out 15ml with a syringe, fill the bottle with it, and you still have the “air gap” area left (again, below the neck line), then you can set your numbers to normal. If no air gap, then you know you need to adjust your numbers down to maybe a 12-13ml mix target.
The aspect of bottles not being accurate is one of my bigger pet peeves. And while I won’t bust the particular vendor’s balls… I will say that I’ve had a proper 30ml bottle order, followed by a second order which are more like 27ml bottles (no gap). And it seems that I’ve also had better consistency with getting proper 30ml bottles in the amber color. Which I hate. So… It is what it is, but like VM alluded to, something that’s not often thought of, but important to be aware of! Having room for off-gassing is critical to a mix steeping well, and in the time frame that’s expected.