JOCKEY WHEELS

I'm not sure the number of speeds makes any difference in terms of jockey wheel teeth - the chain pitch is the same, it's the width that changes.
 
You can use either size but you usually have to space out a 9spd one as they're narrower than the 8spd version. You would be better off using the larger size as the more teeth the longer they last (the force is spread over a greater area thus reducing the force on each individual tooth).
 
...except if you put a larger 11-tooth pulley from a 9-speed mech into the upper position on an 8-speed mech designed for an 10-tooth pulley then it will make setting the mech up correctly harder because the vertical position of the chain would not be quite right with respect to the cassette. You can get away with using a 9 speed mech with an 8-speed chain, but its not a good idea to mix jockey wheels of dfiferent sizes across mechs.
 
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