Answer: 7 at the top
If you bought a bottle of the original formulation of 7 Up, you received a small dose of lithium citrate, much like lithium used in psychiatry to stabilize the mood of patients with bipolar disorder, mania or of depression. Originally called, more transparently, "lemon-lime soda lithiated lithia", the inclusion of lithium was considered a strong selling point for the brand. After all, who would not want a mood boost with each bottle?
7 Up's creator, Charles Leiper Grigg, is he committed to treating his patients? In fact, the inclusion of lithium in its drink is a side effect of where it is installed. Some places in the world have springs with a water naturally rich in lithium, called "water of lithie". When he discovered lithium in the St. Louis water source from which he drew his water, it was natural to ensure that the public knew that his product included the morale-boosting drug and that "Lithiated" was added in the name of soda.
Bonus Trivia: Although there is no official story about the origin of the name '7 Up', the current holding company claims that it refers to the seven main ingredients of origin. One of the apocryphal stories is that the 7 in the name was a nod sign of the atomic weight of Lithium (rounded to an integer of 6.94).