What's the biggest number of notes that a cello can play simultaneously? Does it always play only one note at a time or is it common for a cello to play two lines at the same time? Can it also play three or even four notes, or is it totally impossible? Or is it, perhaps, possible, but very inconvenient for a cellist and, therefore, very uncommon in usual practice?
It's completely possible to play two notes at the same time on a cello, as stated by this website (https://www.8notes.com/school/lessons/cello/beginners-guide-to-cello-double-stopping.asp)
What is a double-stop?
It is simply the playing of two notes on adjacent strings, at the same time. Because the Cello has four strings it is also possible to play triple and quadruple-stops.
So the max is four notes played simultaneously (although this can be hard to do). Playing four notes in a chord is possible by rolling the bow quickly. A cellist can also use their fingers to play four notes at the same time by plucking.
This Music S.E. question should also be useful: https://music.stackexchange.com/questions/20432/is-it-common-to-play-several-strings-of-a-cello-simultaneously#:~:text=Triple%20Stops,tone%20quality%20will%20be%20poor..