Is it possible to ruin a batch by pitching too little or too much yeast?
I figure too little and you will not achieve the correct alcohol content. To much and you will get a batch with a heavy yeast flavor and high alcohol content.
Under pitching will cause low attenuation. Pitching too much can cause off flavors and other issues. Google search yeast starter calculator. Learn all you can about yeast starters.
Under pitching can stress the yeast and cause off flavors with increased production of esters.
This. ^^^
Underpitching stresses out the yeast. They’ll still get the job done, it just takes longer and can allow contamination from wild yeast and bacteria to take hold along with your desired yeast.
Overpitching is dang near impossible, and probably doesn’t hurt anything at all. Much better to “overpitch” if there even is such a thing vs. underpitch.
