From Wikipedia:
There exists a myth that the 7 Up name comes from the drink having a pH over 7. That would make it neutral or alkaline on the scale; however, this is not the case, as the 7 Up pH is close to 3.79, similar to other drinks of the type.[7] The real origin of the name is unclear,[8] though Britvic claims that the name comes from the seven main ingredients in the drink,[9] while others have claimed that the number was a coded reference to the lithium contained in the original recipe, which has an atomic mass of approximately 7. It has now been said that the lithium in the original drink raised the overall ph of the drink to above ph7, dependant on how liberally the lithium drug had been added