So playing Bloodborne finally and..

[Azuren]

Bosses necessary to advance the game:

Father Gascoigne
Vicar Amelia
Shadow of Yharnam
Rom, the Vacuous Spider
The One Reborn
Micolash, Host of the Nightmare
Mergo's Wetnurse

Optional Bosses:

Cleric Beast
Bloodstarved Beast
Witch of Hemwick
Darkbeast Paarl
Amygdala
Celestial Emissary
Ebrietas, Daughter of the Cosmos
Martyr Logarius
Gehrman, the First Hunter
Moon Presence
Ludwig the Holy Blade (DLC)
Laurence the First Vicar (DLC)
Living Failures (DLC)
Lady Maria of the Astral Clocktower (DLC)
Orphan of Kos (DLC)
Abhorrent Beast (Chalice)
Beast-Possessed Soul (Chalice)
Bloodletting Beast (Chalice)
Keeper of the Old Lords (Chalice)
Loran Darkbeast (Chalice)
Loran Silverbeast (Chalice)
Merciless Watcher (Chalice)
Pthumerian Descendant (Chalice)
Undead Giant (Chalice)
Watchdog of the Old Lords (Chalice)
Yharnam, Pthumerian Queen (Chalice)

Gehrman is NOT mandatory for all endings, only two of three. And as you can see, the number of mandatory bosses is actually fairly small. In addition, a number of bosses are repeated in the Chalice Dungeons. These sometimes use alternate versions (Defiled Amygdala, AKA the real Hunter's Nightmare; Headless Bloodletting Beast; etc.) This also doesn't include the optional Hunter Encounters, such as Eileen or Henryk.