Learn Italian: Il museo ha circa cento fossili. - The museum has about one hundred fossils.

circa means "about" or "approximately", so there could be slightly more or slightly fewer than a hundred fossils. quasi means "almost", which would mean the museum has strictly less than a hundred (for example, 95 or 99).

If there are multiple fossils, why doesn't cento change to a plural ending?

In Italian, the word cento (one hundred) is invariable, meaning it never changes its ending. Even if you have five hundred (cinquecento), the suffix always stays -cento.

What is the singular form of fossili?

The singular is fossile. Nouns that end in -e in the singular (like the masculine noun il fossile) always change their ending to -i in the plural.

How would I use circa with the time expressions we are currently learning, like due anni fa?

You can put circa right before the number to express an approximate amount of time. For example, circa due anni fa means "about two years ago".