Learn Portuguese (Portugal): O ano tem doze meses.

The singular word for month is mês. The circumflex accent tells you to stress that syllable and gives the vowel a closed sound. When you make it plural by adding -es to make meses, the normal pronunciation rules naturally put the stress on the second-to-last syllable. Because the natural stress lands in the right place, the accent mark is dropped!

How do you pronounce the two 's' letters in meses?

They actually make completely different sounds! In Portuguese, a single 's' sitting between two vowels makes a "z" sound, so the middle 's' in meses sounds like a "z". Meanwhile, in Portugal, an 's' at the very end of a word makes a "sh" sound. So, meses is pronounced roughly like "meh-zesh".

Why is tem used here instead of têm?

Both come from the highly irregular verb ter (to have). You use tem (no accent) for singular subjects like he, she, or it. Here, the subject is o ano (the year), which is a singular "it". You only use têm (with an accent) for plural subjects, like "they".

Does the number doze change depending on the gender of the noun?