Does Yorick's ultimate "Omen of Death" count as one more summon from his passive or not? Quote from passive:" Yorick takes 5% less damage and his autoattacks deal 5% more damage for each summon that is active."
Yes, having his ultimate summon active also increases Yorick's damage and damage reduction. From the League of Legends wikia:
Unholy Covenant is a passive ability, causing Yorick to deal more damage with his autoattacks and reduce incoming damage by a percentage. The strength of this effect is determined by the number of summoned units from Yorick's champion abilities that are currently active.
Summoned units include the Spectral Ghoul, Decaying Ghoul, Ravenous Ghoul and Revenant.