A unit's number of Eyes is not mentioned anywhere on their character sheet, but it's typically either obvious in the sprite or mentioned somewhere in the unit's description/lore. It's obvious that humans have two eyes, for example, but the Pale Ones have only one.
The "Eye Count" is actually pretty important in combat, as it determines how many eyes you can lose (via Afflictions) before becoming completely blind. Losing an eye reduces your Attack Skill by 2, your Defence Skill by 2, and your Precision by 3, while going blind reduces these by 9. Blindness overrides the cumulative penalty of multiple lost eyes, but it's still as bad as having three lost eyes for long-ranged fellows, or five lost eyes in melee. In a nutshell, those with just one eye are practically asking for the Polyphemus treatment.
On the other hand, units with the Blind trait don't particularly need their eyes, and can't get the Blind affliction. The Zotz have the Blind trait, for example, so getting both eyes sliced is no big deal for them.