I want to know how many creatures/beasts in Skyrim which you have to fight during the main quest, and second question: Are there boss fighting stages apart from dragons?

Thanks

It depends. It may be more than you'd like, but fewer than you'd think. On the other hand, it may fewer than you'd like, but more than you'd think.

I didn' understand I want a clear answer please, can I expect more than Kingdoms of Amular? And are there boss fightings apart from dragons? Thanks

Here you go.

http://www.uesp.net/wiki/Skyrim:Creatures

lol

There are more "boss fights" than dragons. I just had a fight with a bandit leader that was pretty "boss" like. There's others you'll find if you just play the game instead of having other people play it for you.

bears , spiders and more bears and more spiders and even bigger bears and larger spiders

Guess you can consider Forsworn and bandits, and necromancers and vampires, ect... creatures too. If you work in varieties there's a lot of them, but really just generally there aren't that many.

Not enough for my liking...

Manbearpig.

wolves (more than 1 type), bears (more than 1 type), sabrecats (more than 1 type), horkers, mudcrabs, flame atronachs, frost atronacks, storm atronachs, daemora, whispmother, whisps, ice wraiths, trolls (more than 1 type), giants (more than 1 type), mammoths, draugr (more than 1 type), falmer (more than 1 type), vampires (more than 1 type), werewolves, bandits (and other human foes like forsworn, thieves etc), ghosts, dark shadows, wild dogs.
I'm probably missing something but these are the regular monsters I can think of atm

EDIT: dragon priests, frostbite spiders, chorus

Humanoids count as one category. A Forsworn and a Bandit are really the same thing with a different model. A caster of any type and a Bandit however are slightly different in that you need to adjust your tactics slightly. The problem with Skyrim is that the creatures/enemies do not have enough distinction. They don't really have any special attacks or abilities like most RPGs you need to account for and adjust.

Spiders and Dragons and Bears, oh my

There are a few Boss fights. I would say a Briarheart on lower levels is definitely a BOSS, man they kicked my butt from here to Oblivion a few times! LOL
You also have Dragon Priest bosses and some Draugr Overlords. And yea, there are a few Boss Bandits.

i would think there a [censored] load of animals in this game but its more like quantity not quality.

There is a huge lack of variety in creatures in Skyrim. You will fight or at least see every monster or creature in the game by level 10 at the latest.