How many vowels in each syllable?

The link is for children learning to write at age 4. (or rather for their parents and teachers). It isn't really directed at non-native learners.

Children are taught to break a word into syllables and sound out each syllable in order to attempt to spell the word. Syllables in English can be a single vowel or dipthong. "V" or combined with consonants "CV" or "CVC" The "V" may be a single vowel, like /a/ or /i:/ (spelled "ee" or "ea" for example") or a dipthong like /ai/ or /ei/, which might be spelled "i" or "a". However a syllable can't contain two vowel/diphongs separated by a consonant. "VCV" must be two syllables. And single consonants are not (normally) sylablles. Also the consonant might be a digraph ("th", "sh") or a blend ("st", or "bl").

If you look hard, you will find exceptions to these, but this is good enough for a five year old!

So to spell an unknown word, the child is encouraged to break it down and spell out each syllable phonetically. "computer" could become "kom" "pyoo" "ta" and then spell each one: "kompyoota" That is a good first attempt.

Every syllable has one vowel

This is an overgeneralization and is misleading. This may be true in some languages, but not in English.

In English, a syllable can be composed of a diphtong, which is a unit that is a combination two vowels. A long vowel may also be considered as a instance of two vowels.

An English syllable also doesn't always have a vowel since liquids and nasals can be a syllable peak. See link.