I've never had a problem with 1 packet of a Fermentis strain, and since my LHBS wants nearly as much for fermentis as a vial or smack pack, two packs gets to be a bit pricey.
I'm willing to risk it on my batches
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10