I've found one magicshine on the helmet to be plenty for any trail riding, but I throw two more on my dirtbike and the three together seems to put out more than my my streetbike's highbeam. Tested up to around 100km/h on FSR with complete confidence, that's a lot faster than I will ever do on a mtb.
I usually put one on the mtb handlebars too, though, just to have a spare, and it does make riding a bit easier - you can still look way down the trail but see what your wheels are going over out of your peripheral vision. With only one on the lid I find myself looking down at the front wheel a lot, rather than down the trail.
Ordered several times from Geoman, every time I get hit for some fees at the border, the amount seems to vary with who checks it at customs but generally tax/duty/extra shipping fees from Canada Post ?!?? was adding about 10% to orders. Always got a bunch of friends together to order a pile of them at once.
Great service from Geoman too, they have warrantied stuff for me that I didn't think deserved a warranty, despite my full disclosure to them about battery abuse.