OP With the way people are turning to bitcoin right now it's hard to estimate how many bitcoin holders there are. But this much can be said over the past few years, the number of Bitcoin investments and holders has increased manifold. Bitcoin is not legal in many countries so I think it is impossible to know the exact number of investors and holders. Now many people in our country have chosen Bitcoin as a safe asset. Even if Bitcoin is not illegal in our country, seeing the success of Bitcoin, many people in our country are secretly investing in Bitcoin and trying to hold for long term. In the current situation of Bitcoin, this much can be understood, it is expected that the number of investors and holders of Bitcoin will increase several times in the future.
It only be accounted properly if we all are using a single exchange but we have a lot of exchanges available in the market and they won't also speak out.
I don't think it is necessary to know instead, let us assume that the number of Bitcoin holders is growing fast every day.
However, we could also say that even though the number of holders is growing, some also are leaving.
All we've got are just estimated numbers but it doesn't matter because what is important is that we see a growing number of individuals who are in crypto already.