I do about 10 pidgeon steps for canter poles, 5 for trotting poles and 7 for a stride. It varies hugely though as adults have different stride/feet lengths, horses have different strides and collection/extent ion work calls for different lengths. Have a play and see.
It depends if it's a pony or horse. I work in yards as a yard is roughly a human stride. A horses stride is 4 yards ( 4 human strides) however if the quality of canter isn't very good/ and your training I shave a little off and set it out at 3.5 yards so 3 and a half human strides.