Jim, the number ten is mentioned in Exodus 34:28-at the end of the part where Moses goes to Mt. Sinai. Here's the King James Version:

28 And he was there with the Lord forty days and forty nights; he did neither eat bread, nor drink water. And he wrote upon the tables the words of the covenant, the ten commandments.

And here's from the Complete Jewish Bible:

28 Moshe was there with Adonai forty days and forty nights, during which time he neither ate food nor drank water. [Adonai] wrote on the tablets the words of the covenant, the Ten Words.