Anonymity of Paillier cryptosystem in e-voting system
You've discovered some of the challenges of secure electronic voting. As you can see, merely having a cipher that allows you to confidentially tally numbers is not sufficient to conduct a secure election.


  Now if each vote is not signed (or identifiable in some way) how can I exclude that Trudy has not forged a huge amount of fake votes? For that reason I suppose that each vote has to be identifiable when still encrypted.


This is the issue of ensuring that each voter only gets one vote.


  What I can't understand is what prevent the election official to decrypt that votes one by one and see who vote for what.


This is an issue of vote privacy, the inability for even the officials who run the election to know who voted for what.

There are lots of different schemes that  use different techniques and make tradeoffs regarding the goals and efficiency.

E-voting is a subject unto itself. This paper provides a gentle introduction.