It usually depends on the age of the horse, if it is a youngster you usually go twice, for an aged horse who may have some problems, going out once is enough. also the temperature outside and the humidity come in to the equation. if it is beastly hot, you may want to limit how strenuously you warm up your horse. in the US in moderate weather, it is usual to let your horse stand for 2 races from his warm up to his call to post. for instance, if you are in the 6th, and you only are going out once, you would go out after the 3rd race. in very cold weather, they usually stand for one race. If you are going to turn the horse and go a trip, you just back that up and go, stand 1 or 2 races, depending on how hard he is breathing and go again and stand at least 2 before his race. so much depends on the horse's condition and how he best responds that there is not one classic answer for all.