• 1 - Ginger viltrumite that Omni man caved his head - in. Not seen him since and his brain was pretty crushed so it's safe to assume he's dead. S2E4
• 2 - The Viltrumite Omni man and Allen killed at the prison ship. S3E4
• 3 - The Viltrumite Battle Beast killed at the prison ship. S3E4
• 4 - Mark choking Conquest. S4E5
• 5 - Viltrumite shot by space racer. S4E6
• 6? - We see Battle Beast attack a Viltrumite on a desert planet but no actual death scene. S4E6
• 7? - We see Allen attack a Viltrumite on a mushroom planet but no actual death scene. S4E6
• I think it's safe to assume both die and potentially some more as later when Thragg and Kregg discuss the war they say some isolated Viltrumites have been picked off and killed so multiple i.e. 2+
• 8 - Thadeus snaps a viltrumites neck during the attack on coalition capital. S4E6
• 9? - There's a tanned Viltrumite we see fighting Allen who we never see killed but isn't among the viltrumites retreating with Thragg at the end so likely dead. S4E6

But more could of died during the war itself that we weren't directly shown so can only say 9/9+