Say an amount

A number slots straight into the X は Y です frame to state an amount.

JLPTN5

[number] + です

じゅうななさいです (17 years old) or ひゃくえんです (100 yen) — the number just fills the Y slot you already know from X は Y です. Nothing conjugates; a price tag doesn't even need a topic, just the number and です.

Tips and Common Traps

Maru· Common trap

A number plus です is a whole sentence: じゅうななです answers “how many” with no verb and no counter.