Closest answer is Heather and Joe collectively met 20.(if assumption 5 is dropped)
Certain logical deductions-
1. There are 11 couples including heather and Joe, i.e. 22 people.
2. NO person can take the value of 22(meaning he met himself.........lol), 21(he met his partner).
3. Joe noted 'no two guests' and 'none of them' meaning Heather answer could have been repeated because it is yet to be counted.
4. So 21 people (removing Heather) Can take 21 values(0-20)
5. the values 0 and 20 cannot co-exist. Anyone claiming to have met 20 people must have met the person claiming to have met 0 people. Thus, 21 people (removing Heather) Can take 20 values(0-19)or(1-20).
Note- deduction 5. is based on the assumption that the introduction leads to +1 for both parties.
if this assumption is dropped an answer could be reached.