Edward Chang's team used 253 ECoG sensors spaced 3mm apart on brain motor production areas for speech in a 2023 Nature paper.

“So, um, what we did was we did a surgery where we implanted an array of 253 ECOG sensors. These are the sensors that are densely spaced. How many? 253. So, we're talking about something about the surface area of a credit card. And it's filled um with electrode sensors that are spaced about 3 mm apart. Each sensor is about a millimeter in diameter. And so basically you've got this credit card sized array that was placed on the part of our brain that processes words in particular the motor production of the words. The parts that control the lips, the jaw, the larynx, the tongue. Um areas that um were functionally disconnected from her vocal tract because of this stroke and the brain stem which connects the brain to those muscles.”