Willy, I think you mean you have a 16 channel mixer.
Depending on the configuration of the console and if you are using any sort of eight track recorder be it HD, ADAT, or (gasp) reel-to-reel, You use the first eight channel strips to go direct to the recorder and bring the signals back into the board on the other eight channels so you can monitor and mix down.
If you have a switch at the top of the strip right after the trim labled "line in", 'mic", "tape", you use the tape position to listen to the eight signa;s returning from the tape deck. If you do not have this switch or one similar, you need to dig into the manual you have or ask about the specific console you are using.
I'm not at all familiar with Behringer products, but this procedure works with the majority of small studio/stage boards.