A staple charge against Calvinism is that unconditional election is "arbitrary". Isn't "unconditional" election arbitrary by definition? If it's "unconditional," then it must be arbitrary, right? Like rolling dice.
However, that's confused. It's unconditional in the sense that it's not conditional on anything outside of God. But that doesn't mean there can't be differential factors in God's choice. Different human agents, or the same agents making different decisions, will produce different chains of events. God could have reasons for preferring one chain of events over another. And that's still not conditioned on something outside of God, since God is the one who makes different agents different in the first place. The differences derive from God. Likewise, the same agent could choose a different course of action in the way a movie may have an alternate ending. Different plots originate in the mind of God, not the human agent.
But we also need to examine the notion of "arbitrariness". There are degrees of arbitrariness. There's a distinction between what's reasonably arbitrary and unreasonably arbitrary.
Consider age of majority, age of consent, drinking age, driving age, voting age, draft age, enlistment age, &c. Suppose, in most cases, that's 18. Now, that's a somewhat artificial threshold. Why not 17 years, 5 months, and 13 days? Why not 19 years, 2 months, and 9 days? In part because lawmakers like round numbers. In part because we've inherited the decimal system.
Moreover, the age at which individuals are actually competent varies from one person to the next. But, of course, it isn't feasible to have a variable age. It has to be a general policy. Lawmakers can't individualize for millions of people.
But even though the threshold is somewhat arbitrary, it's reasonable arbitrary. To begin with, the alternative to a somewhat artifical age threshold is to have no minimum age for sex, voting, drinking, driving, military service, &c. But that would be ridiculous.
It is, therefore, preferable to have an age threshold, even if that's somewhat artifical, if the alternative is to have no age threshold at all for such activities.
Moreover, 18 is reasonably arbitrary in a way that 8 or 28 would be unreasonably arbitrary. 8 would clearly be too low while 28 would clearly be too high. So the threshold is a rational compromise. A line has to be drawn somewhere, and while there's no ideal place to draw the line–indeed, because there's no ideal place to draw the line–any threshold will be somewhat artificial. Yet drawing the line some places is more reasonable or unreasonable than other places.
Take the age of consent. Take statutory rape. If an 18-year-old seduces a 17-year-old, that's legally rape. Yet that's a legal technicality. The transaction was consensual.
What if the seducer (or seductress) was a day shy of 18? Legally, it's rape if it happened a day after he or she turned 18, but not a day before. The fact that the seducer (or seductress) happens to fall on the liable side of that threshold may be arbitrary. And for that reason, I it would be unjust to charge the seducer (or seductress) in cases where it's nothing more than a legal technicality.
But, presumably, that's not the intended target for age of consent laws. Rather, if you didn't have such laws, you couldn't prosecute someone for bona fide child rape. Or situations like the Roman Polanski case. So you need a certain spread to cover the egregious examples.
So even if unconditional election were arbitrary in some respect, that doesn't ipso facto make it capricious or unjust. A critic of Calvinism needs to fine-hone his objection, to show that unconditional election is unreasonably arbitrary (even assuming that's is arbitrary in any sense).
In principle, God could elect one more person or one less person. But even if there's a sense in which that's arbitrary, it doesn't follow that it's unreasonably arbitrary. If you were to take that objection to a logical extreme, a non-arbitrary line would be to damn everyone!
Moreover, there's a sense in which God can't elect everyone possible person, even if he so desired. For the existence of some people depends on the existence of evil. And in many instances, that evil would be incompatible with the elect status of every ancestor. It's a domino effect. So election necessarily includes some individuals who might have been excluded while excluding some individuals who might have been included. A possible out is of God creates a multiverse. That, however, will entail evil.