How many notes can a cello at a time?

The cello can play 4-notes stops but they must be arpeggiated because of the way the bridge curves. The most noted that a cello can play and sustain are 2. You can also play a 4-note chord by playing the lower 2 notes as if they were grace notes and then the upper 2 notes and sustaining those. Listen to Bach's Cello suites. Great solo cello writing with an illustration of many double and triple and sometimes quadruple stops.