There's a hard limit on the amount of people that own one bitcoin: There currently are only 16.1 million bitcoins in circulation, so there can't be more more than 16.1 million people with one bitcoin. However, there are a number of individuals that control much more bitcoin than that, so it is safe to say that the actual number of users that own one bitcoin is much smaller, I'd guess less than one million people.

On the other hand, the number of users that own any bitcoin at all is very hard to estimate as well. There are currently 46 million unspent transaction outputs, so if we'd assume that every user maintains their own balance that would be a strict upper limit. However, many users leave their coins to be managed by online wallets or exchanges, so it's not a reliable limit. On the other hand, the number of people is probably even much smaller than that, because few people own many of those UTXOs. If I were to hazard a guess, I'd say that there are fewer than five million people in the world that own any bitcoins.

Of ~16 million coins out there, as much as 2.2 million have never been moved or have fallen out of circulation. We're looking at maybe 14 million coins.

Then there are people who for one reason or another have much much more than others.

If there are 11.4 million wallets I'd be surprised if even 1% contain any value. Let's say 10 percent of wallets are a coiners main wallet.

11.4 million wallets 10 wallets/person 14 million bitcoins (unevenly distributed.)

So out of ~1.14 million coiners, they each could have at maximum 12.3 bitcoin, but they don't. I'm sure there are much less than one million souls with at least one bitcoin.

I came across the following in an article

"Nobody knows how many bitcoin users there are but there were 11.48 million bitcoin wallets in existence in January 2017 according to Blockchain.info. Each bitcoin wallet is a potential customer for an online merchant."

So I would say a safe guess is anything less than ~11.5 million. The obvious next question is the average amount of addresses per person and how many people are using cold storage. So essentially 11.5 million / 7.5 billion global population would give us ~0.15% of the population actually owns a bitcoin (my estimate is less)? An interesting indicator of bitcoins potential in the future.

Based on what others have said, we could make a low-end estimate of the number of crypto users to be well under a million active users. If that indeed is the case, it could be a bad sign as it would indicate that the increase in price hasn't been driven by meaningfully increased adoption but rather by speculation. I also bring this up because I noticed the amount of views and subscribers on YouTube crypto channels is quite low. If the cost of coins is already prohibitively high enough to prevent newcomers from adopting/investing and current price trends are driven by whale holders and speculation, we could be setting up for a major downwards correction or even a market collapse.

One thing we all probably know for sure, is that Bitcoin and Alt Cryptocurrencies are getting more coverage in the news, and that coverage is increasing at an increasing rate. My gut tells me that the numbers of individuals getting into Bitcoin is probably also moving along the same graph lines as the news coverage. The price increases only encourages involvement, as small fractional investments will increase at the same rate as large investments, so the little investors may also reap great returns. When that fact is understood by the neophytes, millions of fractional Bitcoins buys will be the norm. This points to the need for faster transactions, and you get where that leads. Lastly, as the base of ownership broadens, the more stable the movement, as far as ups and downs, and the more these small buyers are buying, the price will continue a more sustainable rise. But, all this is opinion based on many years of experience, and it could be flawed. However, I’m an aggressive buyer based on my opinion and gut feelings. When the upside is so massive compared to the downside on new innovations, those are risks indicators worth taking.