The ActiveTwo Base System has no channels. The Base System consists of the parts that every ActiveTwo system needs to have, but excludes the parts that are used to custom configure ActiveTwo for various purposes, like amplifier/converter (AC modules).  The AC modules give the system the capacity to record from active electrodes. With four of the eight channel modules, the base system will have the capacity for 32 channels. If you want 32 electrodes on the scalp, then you need four modules, but if you also want to use four individual flat-type electrodes, you need another eight-channel module (four modules for EEG and one module for EXG1-8). The modules have eight channels, so you can only expand the system in increments of eight channels. You don't have to buy the electrodes to use with EXG5-8, but you need the eight channel module.