In a standard EEG (electroencephalogram) setup, the number of electrodes used can vary depending on the purpose and the specific system being used. However, some common configurations include:

1. 10-20 System: This is the most widely used system for clinical EEG and research. It involves:

(1) 21 electrodes in the basic 10-20 system.

The 21 electrodes used for EEG recording, distributed according to the 10-20 international placement system (Nunez et al., 2006). The clustering into N R = 5 zones is indicated by the colors and dashed lines (1 = frontal, 2 = left temporal, 3 = central, 4 = right temporal and 5 = occipital).

(2)32 electrodes in an extended version for more detailed recordings.

Electrode placements of 32 channels according to the international 10–20 system.

2. High-Density EEG Systems: These systems are used for more precise mapping and may include:

     64 electrodes.

    128 electrodes.

    256 electrodes for advanced brain mapping research.

The choice of how many electrodes to use depends on the application-routine clinical diagnosis typically requires fewer electrodes, while research and high-resolution mapping may use more.