Question

What are the resonance isomers for `CO_2`?

Answer 1

There are `3` resonance structures for `CO_2`.

Explanation:

The website actually also lists `3` resonance structures—the two structures at the beginning are the same thing.

(These are the same thing.)
The only difference between then is how the valence electrons for the two oxygens are drawn, but, in both cases, the number of valence electrons on both is the same.

The three resonance structures for `CO_2` are:

However, only one of those structures is the major resonance contributor.
For more information as to why, check out this great answer by Stefan V.!