In other posts there's similar problems with the playlists, but I couldn't the precise answer about the limit of songs or limit of hours in a playlist in order to be played ALL the files.

I read in a post that a shuffle playlist only plays 6 hours of songs? and If I upload 1500 songs for example, 30 hours of songs, the Centova only "choose"  60 hours or 100 songs?
Is that why the shuffle repeats the same songs?

So, does that mean that any playlist is limited for 6 hours?
So If I have a bunch of songs/hours, Do I create bunch of playlists of 6 hours each in order to play all the files?

That's a tiny misinterpretation 

Centova Cast will do it's best not to repeat the same track within a 6 hours interval, but that doesn't mean it will limit your playlist to just 6 hours of the same songs.

Perhaps this article will clear things up:
http://www.centova.com/en/faq/cast2/information/limiting_performances_of_a_single_track_in_given_time_period