I have no idea how big or small the collector market really is. Relating to US paper money only, how many collectors are there? For this purpose, I'll define a collector as someone who has made more than 1 collectible currency purchase in the last year. It could be small size notes, large size notes, colonials, obsoletes, confederates, stars, errors or fancy serial numbers. I know it's not an easy number to determine, but what's your best guess?
At 317,000,000 people in the United States, I would guess that 2.5% of people are 'active' US currency collectors. That would make 7,925,000 people. I would further guess that there would be about 2,000,000 international collectors of US paper. So, my guess (and this is a rough, rather uneducated guess) would be just shy of 10,000,000 people. I would further surmise that many of these are not serious collectors, but just those who have a small collection - by your definition, purchasing one note per year.
Almost 8 million people collecting even two notes a year seems very high. I would guess 500,000 total in the U.S. market and more like 75,000 who are serious collectors. IMHO.
Hmm. Although I focus on ancients, I would be included in your definition, as I've made three or four buys in the last year. This is one of my favourites.
Based on the number of times I was first underbidder these last two months, I believe the correct answer is TOO DAMN MANY.
When I look at attendance figures for major currency shows, that would mean every active collector had attended. Even if some did come back an additional day. ( Active based on more than one purchase a year.) I was figuring on approximately one in every 5,000 people in the U.S. Maybe one in 10,000? = 35,000
All people collect money. Most spend it, and some keep some of it. Then they spend the rest to buy more of it to keep. When the prices of common currency goes up, and are not rare, then you can say that there are more collectors, collecting currency. How to tell, will be hard to figure.
Using the definition given in the OP, the number is going to be artificially high if you are trying to estimate the number of active collectors. It would include everyone who bought a sheet of FRN after visiting the BEP as well as those who bought a "state commemorative" from an ad in the Sunday newspaper.
That's why I tried to define it as a buyer who has made more than 1 purchase. If someone buys 2 sheets or 2 commemoratives, I'd be satisfied to call them a collector.
I have a collection of a couple hundred different bills, fractionals, silver cert., obsoletes, and more. However, I am currently only purchasing national notes for one particular town. Given the fact that I haven't seen one come for sale in about three years I haven't made a purchase in that time. So am I still a collector?
Go to Memphis on a week end in June can't remember which one you will find lots of collectors. Go to any coin show you will find lots of them there also. Don't know the number but they are out there.
I don't know much about paper money, nor have I ever purchased paper money for collection purposes, but I do look at the serial number on them. I got this one yesterday. Again, I don't know much about paper money, but I suspect it's not worth anything since it's a recent issue and in well circulated condition.