Ah, SpawnFleet is one of those commands that's not really user-friendly. It's more for modders who want to test their faction's fleet makeups.
You can't control exactly how many ships the spawned fleet has, but you can set how many fleet points it will try to take up. A good, large pirate armada would be spawned with "spawnfleet pirates 250 0.6 Armada" (minus the quotes). That creates a 250FP pirate fleet named Armada, which was ~40 ships large when I tested it. The 0.6 is the quality modifier; if you want a tougher fight you can bump that up to 1.