what is its winning percentage?
Between 52 and 56 percent.
First-move advantage in chess
 Since 1851, compiled statistics support this view; White consistently slightly more often than Black, usually scoring between 52 and 56 percent. 
what is first-move advantage in chess? There is a general consensus among players and theorists that the player who makes the first move (White) has an inherent advantage.
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First-move advantage in chess [SEP] Introduction
A few notable players have argued, however, that White's advantage may be sufficient to a win: Weaver Adams and Vsevolod Rauzer claimed that White is winning after the first move 1.e4, while Hans Berliner argued that 1.d4 may win for White. Chess is not a solved game, however, and it is considered unlikely that the game will be solved in the foreseeable future. Some players, including world champions such as José Raúl Capablanca, Emanuel Lasker, and Bobby Fischer, have expressed fears of a "draw death" as chess becomes more deeply analyzed. To alleviate this danger, Capablanca and Fischer both proposed chess variants to revitalize the game, while Lasker suggested changing how draws and stalemate are scored.