What is your clinic's success rate per SET for you age demographic? You can enter your info into a binomial probability applet:
The first number ('Probability of success on a single trial') should be whatever probability of success your clinic gave you for your SET, like if they say you have a 50% chance per FET of a single embryo, then it should be 0.50. If they said 35%, then it should be 0.35.
The second number (the number of trials) should be 5. The third number (the number of successes) should be 2, since you want to know about the chances of two kids.
Hit calculate, then the applet will fill in the probabilities. What you want to know is what is the cumulative probability that you have two or more successes... so, P (X >= 2), the one all the way at the bottom. This is the chance that you will be all set with the embryos in the freezer.
P (X<2), the second calculated probability, is the chance that you will need to have more sperm because you will have less than two successes with your FETs. Granted, this isn't quite the same as needing backup sperm because there's the "what if they all fail" scenario in which you don't need to have more of the same donor, but still, a useful metric.
Perhaps your wife will be swayed by some cold, rational statistics. I WANT to believe that every one of my freezer embryos will be viable and that we will have two more babies and then we will donate three embryos to another couple, but that isn't really likely!