Depending, from where you get your information, you will find out that you can store as many leads as you want or there is a strict number that you should put in two or three pieces at most.
The main worry with keeping more leads inside the pencil is that the leads will break and produce fine dust which will clog the pencil eventually, the mechanism will jam, or the leads will be unusable due to breakage.
Rotring in the product care states that you should refill with two or three leads. "We recommend refilling your pencil with 3 fine leads. It’s the best balance for longer use without creating too much dust inside the tube due to an overload of refills."
Staedtler for example, doesn't mention the number of leads that you should keep inside your pencil, but the refill is designed to dump the entire content in one go. "Simple “12-a-go" refilling for many Staedtler mechanical pencils such as Mars micro 775 and graphite 779"
At the same time, I have a feeling that the leads have improved a lot over the years. The resistance is a lot better, while the quality of the writing (line darkness and smoothness) remained the same if not improved over time. A good argument for my opinion is the lack of the needle that used to be present in all mechanical pencil erasers, used to clear clogs in the advance mechanism,. Nowadays, the needle is missing basically from all modern mechanical pencils. Some producers still keep it around, but it is more and more a rarity.
To see if the recommendations are still valid today, I will do a test for a 4 week with two Rotring Tikky III 0.5. One of them will be loaded with 3 leads (1 in the chamber ready to write and 2 loose in the barrel), while the second one will carry an entire refill,12 leads. I will carry around both pencils and use them in rotation, trying to keep it as impartial as possible.
From the beginning I can feel the mass of leads slushing around in the pencil which holds an entire refill pack, and defiantly they create more noise and commotion compared with the 3 leads pencil.
I feel like I am a master of the mechanical pencil, like badass movie characters that know the difference between an empty gun and a fully loaded gun (Lian Neeson Taken reference)
Ant the results are in...
So from what you can see, the leads came out just fine from both pencils. None of them broke, and no dust came out of the barrel (white paper test). In conclusion, I wouldn't care too much of how many leads there are in a pencil, as this non-scientific tests seems to point that even a big number of leads will do just fine in day to day scenario, stored inside your favorite pencil. Generally, it is a good idea to keep a few extra leads on hand, or in this case in the pencil, so that you don't have to worry about longer writing sessions.