1234567890 is an organization for number digits. All of the numbers joined it, and learned how to do math. Then they teach others how to do math. Everyone hates them, but sometimes they are actually really cool, like when 8 eats people, and 2 participates in police raids, and 5 and 4 (Brothers In Chinese Unluckiness) are outcasts.
1234567890 was first founded 1234567890 BC. Of course, back then, it was a book store, but Nobody liked books back then. And ABCDEFGHIJKLMNOPQRSTUVWXYZ sued, so they closed down. But then they reformed on Number Island (then called Madagascar), and became a math-group.
1234567890 was hated for a very long time, as Nobody was smart enough to do math. Eventually, some of the ancient peoples learned how to add 2 and 2, and made 4 famous. They then honored all of the other numbers. Even Pi, a hated number, was not forgotten (actually, it was, but we wrote in the history books instead of listening to the teacher).
1234567890 became increasingly popular over the next few thousand years, even being sponsored by Albert Einstein and Bill Gates. Numbers are actually one of the most important things in the world. And there are infinite numbers, so there is infinity, which is infinite. I'm not sure what I just said, but it sounds like smart people words, so it must be true.
1234567890 is currently based in Number Island, but there are also lotsa lemurs, and they're always making things difficult. This is why 4 carries a Shotgun (he also carries it to look cool, but he doesn't mention this about it).
Their rivals (and doppelgängers) are the Numberjacks, who are also known as 0123456789.