Supposedly dependent on precipitation, the saguaro cactus usually does not grow arms until it reaches 50-70 years of age. Sometimes it grows one or two, and somtimes six or seven. There are probably many factors that influence the number, but I tend to think it might have to do with the plant being able to keep its balance. These arms can eventually weigh an enormous amount, and it seems logical to distribute the weight more or less symmetrically if possible. The arms on this cactus appear to be around 10-20 years old, so still relatively young.