Learn Portuguese (Portugal): Lavem o alho, por favor.

This is the affirmative command form for vocês (plural you). For regular -ar verbs like lavar, the formal and plural command forms swap the usual a vowel for an e. So, the present tense lavam becomes the command lavem.

Who exactly is being told to wash the garlic here?

Multiple people. Because the verb is lavem rather than the singular lave, we know the speaker is addressing a group of two or more.

How would I give this exact same instruction to just one person formally?

You would say Lave o alho, por favor. To make the formal singular você command, you just use the swapped vowel -e without the plural -m at the end.

Does o alho mean a whole bulb of garlic or just a clove?

Alho broadly refers to garlic in general. If you specifically need a clove of garlic, you would use the phrase um dente de alho (literally, a tooth of garlic). A whole bulb is called uma cabeça de alho (a head of garlic).