In Captain Marvel, the Kree refer to Earth as planet C-53.
Does this number signify anything or is it a randomized number assigned to Earth?
In Captain Marvel, Kree have assigned C-53 to planet Earth. That's what they call it. To them, Earth is just another planet in the universe.
It seems to be a random number assigned to Earth. There is nothing special came up with this number yet. Neither from production team nor in a movie.
As the two fight off some stray Skrulls (led by a hilarious Ben Mendelsohn), Vers starts to realize that the key to her past lies on C-53 (that's the Kree name for Earth — we're not that special).
Unless someone from production clarifies it or comes up with an easter egg, there is no significance of this number.
I researched for fun how C-53 could have any substantial significance.
I came across an article from sciencealert.com that says that so far 53 exoplanets exist in our Milky Way using a technique predicted by Einstein. In addition, Iodine is 53d on periodic table of elements. So that and my idea of the C standing for carbon are eliminated.