I don't think it's a good way of gauging the number of people who use bitcoin. I think a better approach would be counting the numbers of customers that KYC wallets/exchanges (eg. Coinbase) have to get a closer number - and even that would be inaccurate and well short of the actual numbers.
Short?
Actually, it looks like people make more than 1 wallet on webwallets.
Also, how can you have more users than wallets?
Anyway, it's been proved that there's a lot more wallets than people, many just register and let it there without funds. They are not Bitcoin adopters, just people that tested it.
Also, many people open many wallets, especially webwallets, I did it myself. I also tried many different clients on my pc and on my smartphone.
So the actual number is probably very inferior to the actual installs of any software and any registrations of webwallets.
This is why wallets are not a good way to estimate the number of USERS actually holding Bitcoins.
By KYC I meant wallets/exchanges that ask for identification like Coinbase. I don't think you can register two verified accounts on Coinbase as the same person. I'm not talking about regular web wallets like Blockchain.info that allow anyone to sign-up with an email.
Edit: Verified users of an exchange may have been a better way to put it.
The problem with this approach is that those users that are verified are, surely, good users of Bitcoin. Probably traders, or venues owners.
BUT, there are a lot of other users that only use a wallet and surely don't want to register with ID check and stuff.
By the way, I came up with this Google Sheet to see how many Android Bitcoin wallets are installed.
I checked the most used wallets but if anybody got some that I jumped please mention it.
Given that Google doesn't disclose the exact amount of installs anymore, I adjusted to the middle of that min-max installs.
Then adjusted again to 70%, just to cut off some people that may have tried more than 1 wallet.
The result is around 1 million.
I would say 1 million of users have at least 1 wallet installed and using it, even if only to hold some coins or just tried it.
Then, we should add the number of pc wallets installed.
And then there's the webwallets.