Ostensibly, he could take on Goku or Vegeta's base form.

https://dragonball.fandom.com/wiki/Yamcha#Power

In the Galactic Patrol Prisoner Saga, Yamcha was recruited by the Galactic Patrol to defend the Earth from the Moro Corps' invasion. Despite the escaped convicts being empowered by Moro's magic, Yamcha proved able to overwhelm three foes at once. Yamcha then goes on to state that he is one of the three strongest Earthlings. When Goku arrives to take care of Zauyogi, Yamcha tells Goku that he can finish taking care of the other prisoners, calling them weaklings.

https://dragonball.fandom.com/wiki/Moro_Corps

The power of each prisoner varies. When Moro shared some of his energy with the prisoners, during the prisoner's group attack on Goku and Vegeta in their Perfected Super Saiyan Blue forms and Good Buu as the Grand Supreme Kai they were no match, with each being downed in a single hit. However they were capable of taking on Goku and Vegeta's base forms.