What is the meaning of the number 54 in Blizzard's games?

Solution 1:

One of the StarCraft art teams is referred to as "Team 54"

Source:

Solution 2:

The rationale for inserting the number "54" on various Starcraft and Warcraft art assets has not been officially revealed by Blizzard. Historically, the company has been fond of using vague references, symbology, or spoofs to draw various reactions from its ever-excitable fanbase.

As linked in other answers, many fans have conjured up suspect or relatively feasible explanations, but Blizzard tends to neither confirm nor deny these possibilities. The instances of "54" are likely yet another example of this odd habit of Blizzard, and will likely never be fully explained. Unfortunately, this makes your request for an answer to pull from "credible and/or official sources" impossible.