Some sources cite eight, others as many as 10. Even syllables containing a single vowel can contain a diphthong. The rule of thumb is: If the sound moves, it’s a diphthong; if it's static, it’s a monophthong. Each of the following diphthongs is represented by its phonetic symbol.
Writing is the act of recording language on a visual medium using a set of symbols. The symbols must be known to others,. No ancient script, alphabetic or not, had pure vowels before the Greeks. The Greek alphabet even has two vowels for 'e' and two for 'o', to distinguish between the long and short sounds.