From wyeast's site faq:
6. Do I need to make a starter for an Activator?
No. The Activator is designed to deliver professional pitch rates (6 million cells/ ml.) when directly added to 5 gallons of wort. ( <1.060 at 70 degrees). However, if a package is slow to swell, suspected of being mishandled, or if the date is approaching the six month shelf life it is a good idea to build the culture up with a starter. High gravity or low temperature fermentations require higher pitch rates. This can be achieved with inoculating with additional packages or making a starter.
Edit: I should have done the math just in case, the packs have this listed on them:
The Activator™ has a minimum of 100 billion cells of pure, ready-to-pitch yeast, plus an internal nutrient packet. The Activator™ is designed to inoculate five gallons of wort (up to 1.060 SG) providing the pitching rate recommended by professional brewers.