When people ask “how many guns does anyone need?” they usually intend it to be a rhetorical question. The implication (particularly from supporters of gun control) is: one at most, but preferably none.
However, this is actually a good question, particularly for the novice gun enthusiast.
Firearms are tools, nothing more. Each firearm has been optimized for a certain task or series of tasks, just as other tools are.
Some tools are very specialized doing one small thing very very well. Other tools are flexible and do multiple things. I have a screwdriver that has extra attachments in the handle, allowing it to fit a variety of different screws. There are firearms with similar flexibility.
At this point, a gun control supporter might note that guns are different because all they do is kill. This is false. There are quite a few designed purely for target practice. A truthful statement would be that all firearms are designed to propel some sort of projectile.
This sense of purpose is not unique to firearms. Every axe is designed to chop something just as every hammer is designed to hit something. Every screwdriver (by definition) is designed to turn a screw, but the variety of screws mean that one may require a great variety of screwdrivers or ones with removable inserts.
This specialization even exists within various categories. For example, a “hunting rifle” can take many forms depending on what game animal one is pursuing. A .22 rimfire is excellent for smaller game, but utterly inadequate for deer. Even within the same species, the environment the firearm will be used in matters – are you hunting deer in dense woodland or on the open plains?
Similarly, the category of “self-defense” has many different elements to it. Is it for concealed carry? Open carry? Home defense? Defense against what? People require a lot less firepower to put down than grizzly bears.
Let us consider a hypothetical person – say a young woman. She is concerned about her personal safety and rather than lugging around a mattress and claiming to be victim, she decides to take control of her safety by obtaining a concealed carry permit (assuming one is necessary in her state) and opts for a compact .380 which fits nicely in her hand and is lightweight and easy to conceal.
However, she also enjoys target shooting and .380 ammunition can be expensive, so she buys a .22 pistol which she uses to practice. She also can take her friends and co-workers to the range because .22s are excellent to train new shooters.
She likes her .380, but also feels the need for something a little more robust in case of a break-in, so she gets a 9mm. Sure, it has a little more recoil, but being a larger pistol it absorbs it better.
Let us further suppose she likes shooting skeet. A 20-gauge shotgun would do nicely. She can also use it for small game, either rabbits or upland birds. During deer season, she goes up north and uses a 30-06 rifle to hunt for deer.
Even within these categories, there is reason to have more than one example. As she moves forward with her career, she might upgrade any one of these firearms and choose to keep her older purchase as a spare or simply because she enjoys the variety.
In my experience it is very rare for someone to have only a single firearm if they have any enjoyment of shooting. Income may be an issue, but generally once one begins shooting regularly, the desire to try something else and fill one of the niches mentioned above becomes irresistible.
I should also note that there are firearms that are modular - like the screwdriver I mentioned earlier. The AR-15 is perhaps the most famous example of this. One can change just about everything on an AR-15, including the caliber. It is the Swiss Army knife of firearms. That is why it is so popular.
Finally, it bears mentioning that people who painstakingly collect firearms are statistically among the least likely to be involved in a crime. While the firearm-phobes may live in terror of people with an "arsenal" of a dozen weapons, it is the gang member with a single stolen pistol that they should truly fear.