The number of eyes is used to determine if there is adequate space to attach a proposed downguy. 1-eye anchors will have a "smoother" eyelet at the top of the anchor while 2 or 3-eye anchors will have ridges in the main eyelet (similar to your knuckles).
The height stick operator can call out the number of eyes while the main camera operator will be responsible to record that number. When the anchor is buried you should select "BUR."