Learn Spanish (Latin America): La ciudad tiene mucho tráfico hoy.

muy means "very" and is used with adjectives (describing words), while mucho means "a lot of" or "much" and is used with nouns (things). Since tráfico (traffic) is a noun, we need mucho. Just like in English, you wouldn't say "very traffic"!

Why is it mucho instead of mucha or muchos?

When mucho comes before a noun to mean "a lot of", it has to match that word's gender (masculine or feminine) and number (singular or plural). tráfico is a masculine, singular word, so we use the masculine, singular form: mucho.

In English we say "a lot of traffic." Do we need a word for "of" in Spanish?

No, you don't need an extra word here. The word mucho all by itself translates to "a lot of" or "much". Adding an extra word for "of" would be incorrect.

Why does ciudad use the feminine word for "the" (la) when it doesn't end in an "a"?

While it is true that most feminine words in Spanish end in "a", words that end in the letters -dad (like ciudad or universidad) are almost always feminine. Because of this rule, we use the feminine article la.