The letter "x" often represents an unknown number in some Math exercises.

在一些数学练习中，字母“x”通常代表一个未知数。

youdao

An unknown number of demonstrators were arrested.

不知多少示威者被逮捕了。

《柯林斯英汉双解大词典》

A couple of hours after I left the comment, I got a text message from an unknown number.

我评价过后几个小时，收到了一个陌生号码的信息。

youdao

Diakite,who is also known as Toumba, escaped the attack and is still at large with an unknown number of soldiers from the presidential guard.

VOA: standard.2009.12.07

Officials say up to 100 armed militants and an unknown number of students remain inside.

NPR: Pakistani Army Commander Dies in Mosque Siege

About 31 Japanese companies had offices in the buildings employing an unknown number of staff.

CNN: Asia pledges cooperation in hunt for attackers

There could be an unknown number of other sports pitches that have been lost, it said.

BBC: Olympics legacy: How can we encourage kids into sport?