EV and KP had 4 each, so unless thats what your saying and ive misunderstood you, your wrong.
technically speaking, both EV and KP have 10 encounter rewarding loot and having codex entry, thus one may say 10 boss. But i agree with you, there is only 3 real boss in EV
The pylon encounter, is not a boss, you just kill (easy) trash while cycling through symbol to find the combination, the first time i did, i totally expected a boss coming out once we opened the vault, but nothing.
The next encounter is just one on ones, no raid coordination, no strat, no team work.
Those are no boss in my book, but they are indeed encounter rewarding boss loot. That is why i said 8-10 boss, depending how you look at it.
While i do think 8 bosses would be cool, i for one know that i could not do 8-10 bosses in one night, and having to clear 2 nights a week for a raid...
i don't agree, once you have mastered the encounter and harvest a few pieces of loot, there is no doubt in my mind that a competent raid group can clear it in a night. Icecrown citadel had 12 bosses, and clearing in one evening in a pug group was largely doable, once server population got familiarized with the various encounter.
As for hard and nightmare mode, it's another story, but i'll take the wild guess that player willing to progress in hard and nightmare mode, of the current content, does raid more than once a week.