This number often gets misinterpreted. I have seen a lot people assuming this is the number of people who have put money into Star Citizen. Even on some gaming sites in articles this number is given as the number of paying players of the game. In reality this is the number of registered accounts.
The number of total sold ships (including ground vehicles) is currently
1.5 million. Since every game package includes a starter ship, there can be at most 1.5 million people that have put money into their account. In reality, as many of us know here, there are people with a lot more than one ship in their account. Based on that it can be assumed that there are a lot less than 1.5 million individual backers. Many of these accounts have been created for free flight events, reserving account names, spam accounts, grey market and referral shenanigans, multi-accounts, or even just to be able to ask a question about the game on spectrum. Everyone can register an empty account right now and be counted as a new citizen.
The best number we have is from
Turbulent in July 2016. They revealed back then that there are 500,000 paying backers. At the same time the number on the RSI site that showed those who "have joined the universe" was at 1.4 million and 1 million ships in accounts. Again, we are now at 2 million citizens and 1.5 million ships in accounts.
The number of individual backers is not published by CIG and can only be guessed based on this information. We may be closing in on 750.000 actual backers currently if growth has been consistent.