The first and second familiars will each spawn after taking damage 15 times. The third and fourth familiars will each spawn after taking damage 30 more times. No further familiars will spawn. As a result, after taking damage 90 times, four out of eight possible familiars will have spawned.
Isaac will look progressively more pregnant as he takes damage.
Lilith starts with this item by default, even if it hasn't been unlocked. Lilith can get a second Incubus through this.
Only one of each type of familiar can be spawned.
Familiars that have yet to be unlocked can still spawn.
Only actually getting damage counts towards the payout. For example, disrupting the Holy Mantle shield while keeping your heart containers full will not spawn any familiars.
Familiars spawned by the item do not affect item pools, so multiple copies of one familiar can be obtained. For example, a second Incubus can be purchased after Cambion Conception spawns one.
If a familiar obtained is a copy of another one you've already obtained, using Box of Friends will only clone one of that familiar, similarly to the Box of Friends' effects on cloned familiars that it spawns.
If this item is removed with XVII - The Stars?, it will also remove any familiars already obtained through Cambion Conception.
Caffeine Pill: Makes it impossible to tell how far along the pregnancy is.