There isn't a perfect answer to that question. In fact that is probably the wrong metric to focus on.
I know you're probably thinking this is not true because so many gurus focus on this metric. But it's more important to tweak it a bit and focus on the cost per sale.
If your goal is to get as many people on your list as possible at the lowest possible cost per lead this can hurt your results.
Wouldn't you agree that having a bunch of people who have no intention of buying on your list won't get you to your goal?
There's nothing wrong with having a big list and that's not the argument. The point is to have a focused target of buyers.
The number of people on your list is far less critical than the number who actually buy from you. Get buyers on the list then scale it up.
An important factor is to understand exactly who you are looking for and how to attract them. Once you have that nailed down you can get as many of these people with the lowest possible cost.
After you get that dialed in you should see your cost per sale go down.