Just a smaller infinity. If there are truly infinite realities, that means even if something has a 0.0000000000001% chance of happening, it still will happen an infinite number of times. There is no single instance of anything, just ratios.
Which is why travel between them is impossible in that set up though, but gets ignored for fiction. If you made a trans dimensional device, that means an infinite number of yous did too. So even if only a fraction of a fraction of a fraction of them end up traveling to the same reality, that means an infinite influx of mass from all the yous that warp in