You could say that as well, but "det är" is more commonly used at least with egg boxes. I'm having a hard time trying to figure out why that is.
"Det finns tolv ägg i kylen." would be the correct translation of "There are twelve eggs in the fridge."
"Det är tolv ägg i kylen." would work as an answer if someone asked you "Hur många ägg har vi i kylen?" Otherwise I thing it would be a little more of the surprised finding e.g. someone put 12 eggs in there without you knowing. "Det är en elefant i kylen." would sound about as strange.