I've had mild effects from 10 seeds, I've had mild effects from 60 seeds. I do think part of the trick is proper preparation, I usually powder them, let them soak in some cool acidified water for AT LEAST a few hours in the dark and then drink the lot; my major failures were when I skimped on any part of that procedure. I've heard letting them sit a day is best, but I haven't been able to do that.
As much as I would love to try really fresh ololiuqui, if you're looking to explore LSA I've found HBWR to be a more reliable source of the magic if you're ordering online. I say "the magic" because that's what is missing from old seeds-- old seeds may still give you a bad drunk feeling, but fresh seeds can take you for a hell of an entheogenic ride. Though of course with HBWR, make sure to scrape off the coating, and don't go above 12 seeds unless you're really experienced with them!
One recommendation I have, even with ololiuqui, is that you have some weed on hand to smoke in case you start feeling generally ill. It'll take the unpleasant side of things away in an instant!