You can make as many wagers, and wager combinations, as you'd like on a given race.  There is a maximum wager amount of $500 per wager, but other than that, you can wager to your hearts content!   

Keep in mind, the harder the outcome is to predict (such as "First 2"), the higher the return on that wager is going to be.   So while your likelihood of a return is much better on a show wager, the amount you'll make off that wager will reflect the risk.