How many people use the Linux kernel? Nobody knows exactly, but it's on the order of billions, thanks to Android and a very large number of embedded devices.
How many people use the Firefox browser? Again, nobody knows exactly, but there's some numbers that can guide you in guessing: download numbers from the Firefox home site, and aggregating user-agent statistics from a large number of popular websites.
How many people use the Koha integrated library system? It much less popular than Linux or Firefox, but it's also something whose usership is somewhat restricted: it gets run by libraries, and you could find a list of libraries (public and private) and ask them if they use Koha and if they do, how many patrons they have. This would probably give you an order-of-magnitude estimate.
How many people use your own software? It's an interesting question for most free software developers, but it's not an easy one to answer. Free software can be shared freely, and there's no requirement for users to register (if there were, that would be a limitation on their freedom and privacy). As outline above, in some cases it's possible to find some actual numbers to use as a guide for estimation. In each case, the numbers come from different sources, depending on the type of software.
A few generic ways to get those numbers exist:
You can get the software packaged in Debian or Ubuntu or another operating system that has something similar to Debian's popcon. This is an opt-in, voluntary system that gathers anonymous usage data about software installations. It provides a rough lower bound for users of software, but anything more becomes guesswork, since only an unknown, but small fraction of Debian installations participate.
You can measure the number of downloads, bug reports, and other interactions with your users.
You can construct search engine queries that find mentions of your software, and see how many hits you get.
You can run user surveys, and see how many responses you get.
You should ask yourself, why does the actual number of users matter? Does it affect your livelihood? Does it make you feel better about yourself? Does it help you justify all the time you put into the software?
Ultimately, you'll have to resign to not know.