Good question. Of the merit badges my son has started or finished:
Whether or not the MB requires some form of contact of an adult (and likely) stranger:
Citizen in the Nation - yes
Climbing - no
Collecting - no
Cooking - no (a bit of a surprise)
Dog Care - yes
Family Life - no
First Aid - no
Gardening - no - requires a visit to a location but no required interaction.
Home Repair - no (and the requirements of this one needs to be fixed otherwise there is virtually no way this is completed correctly unless the counselor is a parent).
Leatherwork - no, must visit a place of business but no required interaction.
Music - no
Physical Fitness - no, requires a doc visit but no interview.
Reptiles and Amphibians - no
Salesmanship - yes
Scholarship - yes
Skating - no
Not a lot of professions on that list so this isn't likely representative but still only 4 solid yes' out a list of 16. I would guess that less than half of all merit badges have a requirement to speak to a stranger. Probably quite a few that require learning about a profession or visiting the place of business without actually requiring the scout to speak to someone working in that profession (e.g. Gardening and Leatherwork)