To explain how invasion works, I will explain it in Halo tense for it to make somewhat easier to explain and understand.
Note: All enemies will constantly spawn over and over when killed.
5 Grunts spawn.
Every 3 Grunts killed, 1 Grunt Major spawns.
So 6 Grunts are on the battlefield.
Every 10 Grunts killed, 2 Jackals spawn.
So ??? Grunts and 2 Jackals are on the battlefield.
Every 10 Jackals killed, 2 Jackal Majors spawn.
Every 20 Covenant are killed, 1 Brute Minor spawns.
Every 25 Covenant killed, 1 Ghost spawns.
Every 2 Ghosts destroyed, 1 Banshee spawns.
Every 10 Butes killed, 2 Brute Major is spawned.
Every 25 Brutes spawn, 1 Brute Chieftain spawns.
Every 15 Covenant Major kills, 2 Elites spawn.
Every 5 Elites killed, 1 Major Elite and 1 Ranger Spawn.
Every 15 Elite kills, 1 Elite Ultra spawns.
Every 30 Elite kills, 1 Elite General spawns.
Every 50 Elite kills, 1 Elite Field Marshall spawns.
Every 50 Infantry killed, 1 Ghost spawns.
Every 10 Vehicle kills, 1 Wraith spawns.
Every 50 Covenant kills, 1 Banshee spawns.
Every 100 Covenant kills, 1 Wraith spawns.
Every 250 Covenant kills, 1 Seraph spawns.
Every 500 Covenant kills, 1 Scarab spawns.
Every 750 Covenant kills, 1 Corvette spawns.
Every 1000 Covenant kills, 1 Battlecruiser spawns.
So on and so forth... so as you can see the more enemies you kill the more will eventually appear. This is not the final listing either. It is just a rough idea.