By the way, what's a Second Seal and how rare are they?
The Second Seal is an item that functions just the same as a Master Seal is used for promoting a unit to a more advanced class once they reach a high enough Level. However, unlike the Master Seal, Second Seals also provide the ability for a unit to change to a different class that is not necessarily mandated by what their class is.
What this means is that you could use a Second Seal on Chrom, when he's at least a Level 10 Lord, to change his class to Cavalier or Archer as if he were promoting to them from Lord, rather than Great Lord. From there, you could use a Master Seal at the appropriate time to change Chrom's class to what would normally be reached from Cavalier (Paladin, Great Knight) or Archer (Bow Knight, Sniper). Chrom has access to all of these classes, and the classes that any unit is able to have using a Second Seal is determined by their character, meaning that Chrom can't be a Thief or Priest no matter what.
Now, using the same example, if Chrom is of any promoted class under Level 10, he can use a Second Seal to go back to the corresponding unpromoted classes as if he were promoting to them (demoting, really). This is done so that Chrom can become different classes and in turn be able to learn the Skills associated with those classes that he wouldn't normally get under his initial class. For example, Level 6 Sniper Chrom uses a Second Seal to become a Level 1 Cavalier. From there, Chrom can learn the Outdoor Fighter Skill and the Skills he could learn from being a Paladin or Great Knight.
Same example again, if Chrom is of any promoted class over Level 10, he can use a Second Seal to go back to any corresponding class. Level 18 Great Knight Chrom can instantly become a Level 1 Great Lord with a Second Seal.
...Got it?
And the campaign gives you a couple, I think. Either way, they're infinitely available to the player as long as they can be afforded. You're supposed to be able to grind for whatever you want.