Learn Spanish (Latin America): Tengo mucho estrés en el trabajo.

Muy means "very" and is used right before adjectives or adverbs (like "very good"). Mucho means "a lot of" or "much" and is used with nouns. Since estrés (stress) is a thing (a noun), we use mucho to say "a lot of stress".

How do I know whether to use mucho, mucha, muchos, or muchas?

When it describes a noun, mucho has to match the gender and number of that noun. Estrés is a masculine, singular word, so we use the masculine, singular form: mucho. If you were talking about "a lot of energy" (energía, which is feminine), you would say mucha energía.

Why does it say en el trabajo for "at work"? Doesn't that mean "in the work"?

Exactly! In English, we just say "at work" without the word "the". But in Spanish, it is very common to include the word for "the" (el or la) with places. So en el trabajo literally translates to "in the work" or "at the job", which is the most natural way to say "at work" in Spanish.

Why does estrés have an accent mark?

Estrés is a loanword from the English word "stress". Spanish adds an "e" at the beginning because Spanish words never start with an "s" followed immediately by another consonant. The accent mark is added over the "e" to tell you to emphasize that last syllable when you say it out loud.