Theoretically every decision you make is related to every race, as if you assume you are investing a finite amount of money, every horse you use is at the exclusion of another horse somewhere on the ticket. Thus, if you find a particularly puzzling race, you will obviously want to use as many entrants as possible, and thus have to figure out how to work that into your play.
On a simplistic level, suppose the race we are discussing has 8 horses, and let's also suppose there is another race where you are undecided about using 2 or 3 horses. Well, you can use all 8 and 2 in the other race for less than 7% more money than 5 and 3 in the other. Thus, you face a choice, and this may be one of the ones that decide whether or not you win.
Obviously that is an oversimplification, as there are six possible legs, but I just use it as an example. With multiple tickets, As - Bs - Cs, you can find ways to use many in a race while minimizing your investment.
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