"赛马场上的赌客" can be translated into English as "bettors at the racetrack" or "gamblers at the horse racing venue".
Both translations convey the meaning of individuals who place bets at a horse racing track. The first option, "bettors at the racetrack," is more concise and commonly used, while the second option, "gamblers at the horse racing venue," provides a bit more detail about the location. Choose the one that best fits the context of your sentence.