の数が

the number of …

→ の
phrase Uncommon #14,814
the number of … Attaches to a noun to make “the number of [noun]” the subject of the sentence.
JA: 参加者の数が少しずつ増えています。
EN: The number of participants is gradually increasing.

旅行者の数が近年大きく増加した。

The number of travelers has increased significantly in recent years.

大学生の数が増加している。

The number of university students is increasing.

奇妙にも、学生の数が毎年減ってきている。

Strangely enough, the number of students has been decreasing every year.

海外へ留学に出かける学生の数が毎年増加している。

The number of students going abroad to study is increasing every year.

最近車の数がずっと増えてきた。

The number of cars has been steadily increasing lately.

生徒の数がだんだん減ってきた。

The number of students has been gradually decreasing.

外国へ行く人の数が増えてきている。

The number of people traveling abroad is increasing.

心臓病で苦しんでいる人の数が増えてきた。

The number of people suffering from heart disease has been increasing.