One ara ara is enough to make every single being within a radius of 5 km orgasm. In fact, one ara causes exactly one orgasm, so it's a double orgasm. Sometimes there is more than two aras, which results in multiple orgasms.
Remember that when you suddenly collapse out of sudden pleasure, Yukarin is near. Don't forget to ask for a portal to Gensokyo. If you are unlucky(?) though, it's another ara ara ojou-sama.