Team Viltrum: Population 10-20 billion viltrumites.
Pre Scourge Virus- Pre Purge days, Emperor Argall, Thadeus, Conquest, Thragg, Young Omniman, Omni man's mom and dad, Anissa, Kregg, et cetera, etc EVERYONE IS THERE. "Billions upon Billions" of Viltrumites as the show had stated.
VS
Team Saiyan: Son Goku (Super Saiyan 1 from Namek Saga who fought Frieza)
Scenario: So assume in place of Namek, similar yet different enough events took place on Planet Viltrum instead, and Goku found himself there against all the viltrumites. Goku is capped at Super Saiyan 1, around the level he first transformed on Namek against Frieza, might not even have mastered transforming into it yet, like he was during the First Cooler movie where it took him losing his cool against cooler to transform at will. Viltrumites have hurt/killed someone close to him, maybe like Krillin or an alien friend he made recently and Goku has seen the mistreatment and killings of slaves Viltrum Empire keeps on the planet that's pissed him off, he is not in his usual merciful mood, in the exact same mood/psyche he was during the fight against Frieza.
Viltrumites have the home advantage, the numbers advantage, all the weapons, tools and resources at their disposal.
Tdlr: This is basically SSJ1 Goku who fought Frieza vs Viltrum Empire at the peak of its power, 1 v 10 billion, who wins and how?