I think the answer is no. but asking anyway.
Is there a way to just download one instrument from a large library from a torrent?
Example: "Mega Brass horns kontakt" includes ; trumpets, saxophone, piccolo, etc. Well, if I wanted to download just the Piccolo, could I? because of course, when selecting the Piccolo, I don't know what the samples of that instrument are, but maybe someone has found a way.

 

Yep, the answer is no almost always. You can download only the instruments you want, even the required nkr file, the nicnt etc. But almost always the samples are scattered inside one (or two) 2GB nkx files

 

and will there be any way to know what that file is without downloading it first?, I mean the 2gb sample package

 

It seems you can (maybe). Download the nki files (along with the nicnt + *.nkr files) and try to open the nki file in kontakt. A window will pop up which will tell you what samples are missing. In the second column of that dialog you will see the path name of the samples which might include the missing nkx containers.

Just try this for yourself with any other kontakt library that you have on your hard drive. Rename the "Samples" folder of any library and open some included nki file. In the "Missing samples" dialog list you'll see also the names of the required nkx files.

 

It is a good idea.

But as I know at the time of downloading that the sample package corresponds to the instrument. Do you think it will go in order? or they will be scattered throughout the packs.
That is, suppose the library has this order

Saxophone
trombone
piccolo
trumpet

and the packs are something like

Brass_complete_01

Brass_complete_02

Could it be that Brass_complete_01 corresponds to the first instrument, in this case it would be saphone and so on? or could it be that the saxophone samples could be in Brass_complete_07
or distributed throughout the library?

 

it could be anything and is up to the developer. If there is no obvious pattern for the nkx filenames then the samples might be anywhere.
Some manufacturers separate nkx files by category (e.g. Flutes01.nkx, Fluets02...; Trumpets01.nkx, ..02.nkx, ...) or bit depth (e.g. Samples_16Bit_01.nkx, Samples_24Bit_01.nkx, ...). If you don' see such a pattern you're out of luck