Reading of the number 10 as とお or じゅう?

Question

I found this sentence online and it had no reading to it. So I presumed that “10” was read as とお considering how this number is counted within the つ counter group.

交わした10の約束は1つも守らなかった。

However, I wasn’t sure so I asked a Japanese person whether it was really とお or maybe じゅう and they said it was indeed じゅう, but they couldn’t explain why. I was wondering if anyone knew? Is it perhaps that no counter is necessary in this sentence? If the number in the sentence was 9 for instance, would it be read as きゅう or 9つ as in ここのつ?

Thanks in advance

Answer 1

As you know

いち - ひとつ
に - ふたつ
さん - みっつ
し - よっつ
ご - いつつ
ろく - むっつ
なな - ななつ はち - やっつ
きゅう - ここのつ
じゅう - とお

There are corresponding words. And in your example, the latter one is already decided ひとつ then the beauty of consistency, it should be とお.
But sometimes とお is not recognized quickly and doubtless unlike じゅう, とお has some homonyms and they are a little difficult to distinguish from another, especially in speech.
So the reason above, じゅうのやくそく sounds like a very common phrase, most people do not mistaken. I prefer じゅう when I talk and I prefer とお when I read it together (in a class or meeting).

Answer 2

じゅう and とお are On Yomi and Kun Yomi of the kanji 十

and it depends on where you are using it:

for saying 10 people or 10 things or 10 o'clock or 10th of the month, you use じゅう

but for saying, for example, the 10th day: [十]{とお}[日]{か}