There are no individual pens in the world. If you've ever been exposed to someone trying to explain quantum mechanics, then you may understand what I mean when I say that pens exist as a quantum wave of "penness" that occasionally collapses into an event that gives the illusion of being an individual pen. I know this for a fact. An individual pen exists solely because the observer is observing it, and the moment that the observer's attention goes somewhere else, the pen will vanish back into the waveform from which it originally materialized.
This phenomenon is by no means limited to pens. Cigarette lighters, paper clips, socks, keys, and just about any item made by the BIC corporation is subject to this - the probability that something will expand into a waveform is directly proportional to the probability that you won't miss it when it vanishes. There are other laws that this phenomenon follows, and understanding them helps to prevent a condition where you know you just purchased a dozen pens, but you can't find any of them.
Increasing the "real" density (number per square foot) of something in an environment proportionally increases the waveform density. A waveform's density determines how often an instance will coalesce out of the waveform. If you saturate your waveform density by purchasing a thousand or so of something, then you can guarantee that there will always be one available when you need it. The number of instances necessary to saturate a wave form for an area is called a "critical mass".
When you have a critical mass of an item, then you will find that there are specific places where the waveform is more likely to collapse. The shelf near the telephone, or a small table near a central doorway, for instance. These locations are called "strange attractors", and their formation is the primary indication of saturation. When you have achieved this, then you will almost always be able to go to that strange attractor when you need one, or when you need to refill the pen cup where they belong.
You can also keep the instance from expanding into a waveform by creating a mechanical form of observation - an inventory. Key rings are a good example. Since all keys are bound to the unit, and the inventory of keys is easily memorable, it is almost impossible for individual keys to vanish from the key ring. Obviously, if the number of keys on a ring becomes extremely large, then this negates much of this advantage. By binding a group of keys together, you form a larger unit that must expand into a waveform as a whole (still not impossible, believe me) or not at all.
An inverse law is that the denser your waveform, the quicker entropy will cause it to leak away. Leakage increases with density until saturation is reached, and then increases much more slowly. Nonetheless, having a lot of pens floating around freely is a good way to have them vanish in large numbers.
Marketing and sales companies have been aware of this phenomena for years. This is why you will never be able to purchase an individual paperclip. Even with the tightest inventories, individual pens and other "quantum macro particles" will demonstrate a tendency to dribble out of existence. They refer to it as "inventory shrink", and blame it on theft, but we know the real reason for it.
If you happen to be the loved one of some poor sod who is far too beset by reality to keep track of every individual pen, cigarette lighter, and set of keys that he or she might happen to own, I suggest the following strategy: inventory what's essential, have plenty of those things that aren't, and remember that the only thing staving off entropy and certain death is your continued patience.