I counted 5 maybe?
Light-hammer ghru guy
Arms guy
Beast guy
Lighting dragon
And maybe the red-haired lady but I'm not a 100% sure about her.
I wouldn't be surprised if the rest you think are bosses would be in fact mini-bosses (like the roaming giant colossus). But I agree with the sentiment, I'm still confused that they showed so much.
Let's see, in order of appearance.
1. Centipede with a Crown guy (maybe a common enemy or miniboss)
2. Scorpion tail guy
3. Werewolf armored Guts guy
4. White two wings Dragon
5. Red haired lady
6. 60 Arms guy with a giant Axe
7. Guy with telekinesis red/black (maybe mini boss)
8. Flying Dragon (only one wing)
9. Smough with Vessel guy
10. Giant without a arm
11. Flying horned creature
12. Shield and Huge sworld guy (maybe common enemy)
13. Giant hand with fire
14. Giant face with wheels
15. Viking guy
16. Wings armored guy
There are more from the sceenshots on the From site too, like a Giant with a white Veil running in the forest and the weird guy on the back of horse. Also, that giant yellow Hand maybe be something too.
Edit:
The post above said that 12 and 16 maybe is the same guy, so 15.