You're contradicting yourself through sloppy thinking. If everyone has another x+12,000 dollars, the money supply has increased. That's the definition of money supply.
It's also a total strawman and not what UBI proposes to do. For example, my wife and I make about $150,000 combined. UBI would basically do nothing for us because of the taxes we'd pay to support it. If anything we'd have less money, not $12,000 more.
The only people who'd have $12,000 more are the totally unemployed and homeless.
The point of UBI isn't that everyone gets more money, it's that everyone is guaranteed to have the means not to starve or be homeless at a minimum.
People who support UBI (including myself) do so not because they would have more money (as stated, I personally would almost certainly lose money), but because they would have less RISK. That's what UBI really does. It lowers the risk of life choices and gives people the freedom to make choices that otherwise they could not have made.
If I'm one of the lucky (which so far, I have been), UBI does nothing for me. But if tomorrow I'm crippled in a car accident and can no longer work, UBI guarantees a certain minimum standard of living.