Is there a centillion of anything?

electrons, protons, neutrons, any sub atomic particle most likely, but we don’t have an efficient way to count and check.

I’ve thought of a new number, with 304 zeros behind it, I call it an XOIIOlion.

Mosquitos or ants or other tiny critters.

No, there is not.

The smallest possible measure of volume is called the Planck volume. This comes from some kind of advanced physics. The Planck volume is about 4×10^-105 cubic meters.

The upper bound on the size of the universe is about 1×10^113 cubic meters.

So, there are about 2.5×10^217 Planck volumes in the universe. What this means is that no object (even if it is as small as allowed by the laws of physics) can fit in the universe 10^303 times.