It will tell you in the unit's description if they can take Necrobionic's, and how many they may take. When with a squad, they follow the same rules for equipment: all must take the same Necrobionic.
Here are some examples.
Initiates and Sacristans may take up to 1.
Necromagus may take up to 2.
Technomancer may take up to 1.
Hope this helps.
SP