Hmm? Modern Seoul dialect only has seven simple vowel sounds (ㅏ/ㅓ/ㅗ/ㅜ/ㅡ/ㅣ/(ㅐ=ㅔ)), not counting semivowel combinations. Even the (largely unrealistic) "standard pronunciation" only has ten vowel sounds (ㅐ and ㅔ are considered different sounds, and ㅚ is considered a simple vowel instead of a diphthong).
So it actually has less vowels than English.
By the way, any scholar who still claims that modern "standard" Korean has vowel length distinction hasn't been paying attention for the last 50 years or so.
cf. https://en.wikipedia.org/wiki/English_phonology#Vowels