Learn Italian: Il museo ha diciotto stanze. - The museum has eighteen rooms.

It comes from combining ten and eight. In Italian, numbers 11 through 16 put the "ten" part at the end (like undici for 11). But for 17, 18, and 19, the "ten" part comes first as dici-. Since otto starts with a vowel, they just click together perfectly as diciotto.

Does diciotto change its ending because stanze is feminine?

No. Unlike adjectives, cardinal numbers (the numbers we use for counting) do not change their endings to match the gender of the noun. It is always diciotto, whether you are counting feminine stanze (rooms) or masculine musei (museums). The only number that changes for gender is 1 (uno or una).

Why does ha have an "h" if it isn't pronounced?

The letter "h" in Italian is always silent, so ha sounds exactly like the vowel a. The "h" is kept in the spelling here simply so we can tell the difference in writing between ha (he/she/it has) and the short preposition a (to/at).

Why does stanze end in an "e"?

Stanze means "rooms" and is the plural of stanza (room). In Italian, feminine nouns that end in -a in the singular change their ending to -e in the plural to show there is more than one.