Antiparticles have the same magnitude of charge as their corresponding particles but with opposite sign. Therefore, if an antiparticle has an atomic number of 8, it means it is the antiparticle of an element with an atomic number of 8, which is oxygen (O).
Oxygen normally has 8 electrons since the atomic number represents the number of protons in the nucleus of an atom, and in a neutral atom, the number of electrons is equal to the number of protons. So, the antiparticle of oxygen would also have 8 electrons.
It's worth noting that antiparticles are typically represented with a bar or a tilde on top of the symbol of the particle they correspond to. For example, the antiparticle of an electron (e-) is called a positron (e+).