Could anyone please tell me, how many skills I have to learn properly in order to take this exam for the certificate (with flying colours)? I am a real beginner... :)
yes, you can take it at any time, and as often as you would like, but as OldSpiceGuy said you might not score very high at first.
I think a few users take it regularly to track their progress.
The test for the certificate is comprehensive, and covers everything Duo teaches about Spanish.