Do muslims generally trust in fatwas or can it sometimes be the case
that there are fatwas from people who just don't know much?
This depends on the source and the particular "Muslims" you are asking about. In general, legal verdicts of Sahabah, Tabieen, Imams of fiqh and most classical scholars are trusted, and even if their opinion is not accepted it is respected. A view that is adopted by the consensus of the early generations of the Muslims is considered Ijmah. Ijmah, in Sunni Islam, is evidence in itself and is considered binding to act upon.
Verdicts and views of later scholars might be trusted depending on their qualification and recognition. Answers on the internet or television range from highly credible to outright heretical.
How can I securely tell that fatwas always come from people who know perfectly what
they are talking about and that the knowledge is perfectly align with the Quran?
You look at the source from where you get that "Fatwa". You can also check if other trustworthy sources corroborate what is being said.
If it is from some particular scholar or his followers, then you can research what opinion their peers and most other Muslims have on them, as rogue scholars will usually face a fair amount of opposition and controversy. Unfortunately, rogue views may also sometimes enjoy popularity on the media.
And are fatwas really nummerated? So is the fatwa nr. 100 for instance the same all over the world?
There is no unified numbering of fatwas, as they are not issued by a single authority. If you find a reference to a number it is likely referring to its numbering in some particular compilation of the fatwas of some particular scholar or body.
And is there a good and reliable website I can take as a source if I want to search for a fatwa?
I find IslamQA,
the answers (not articles) on IslamWeb and the answers on
SeekersHub to be usually reliable. Their answer will be correct, or at worst they would choose a particular legitimate opinion over other legitimate opinions.