The answer is in the description of the offer
For example:
Get a one-time $50 statement credit by using your enrolled Card to make a single in-store purchase of $250 or more at PAUL SMITH by 12/15/2014.
vs
Get a one-time $25 statement credit by using your enrolled Card to spend a total of $100 or more in-store at Sony and online at store.sony.com by 2/3/2015.
Obviously, if it says "a single purchase" then it has to be a single purchase. If it says "a total" then all your purchases are added together.
Most offers are a total, so they don't need to be 1 transaction.