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, negating damage with the Holy Mantle will not count.