The 20th Maine was a regiment though, where the heading here is units/companies. It's a bit unclear what's meant by 'unit' in the heading, which could mean a bunch of possible size groupings, but company refers to a smaller unit, of which a regiment would normally be composed of 10. So 20 regiments would mean about 200 companies.

Some more clarity on what unit sizes are actually being represented would probably make this graphic a lot easier to properly appreciate.