It is controlled by this code part in mjpgStart
if [ ! -d "../etc/webcam_ports" ]; then
mkdir ../etc/webcam_ports
echo "8079" >../etc/webcam_ports/last_port
fi
if [ ! -f "../etc/webcam_ports/last_port" ]; then
echo "8079" >../etc/webcam_ports/last_port
fi
easiest solution is to delete all files in /usr/local/Repetier-Setup/etc/webcam_ports/ and
then run echo
echo "8090" > /usr/local/Repetier-Setup/etc/webcam_ports/last_port
to make it start at 8091 for example. Run startAllCams again to assign the ip numbers.