Vowels 8 and 15, /ʊ/ versus /ɔɪ/ 3 pairs [bullboil.html]
Only three pairs emerged from the computer's search. There is no particular reason for there not to be more. It is a contrast between a short vowel and a diphthong with fairly close starting positions but a difference of lip-rounding. I have not heard of it being a problem to any learner.
The density figure is too small to be worth calculating.