The number of people with Covid-19 has dropped to ○ people.
コロナ感染者(数)が○人まで減った - The number of people infected with Covid-19 has gone down to ○ people. になると思います。
The number of people with Covid-19 has dropped to ○ people. も使えます。
To を使うと「そのポイントまで」を表現することができます。
E.g.
The number of people with Covid-19 has gone down to 20 people.