That depends on the size of your aquarium. Don't put more than one for every nine square inches of aquarium space. Watch their health closely. They let out dangerous toxins when they die.
Depends on the size. Both on the anemone and the tank. 50 gal. per anemone should be fine assuming a medium size critter. Make sure the ones you have in the tank is compatible with each other as well as the current fish and coral.