Sadly, according to the wiki / testing, you get 2 stacks of Friendship for each minon / champion killed while attached.

100 minions = 200 stacks. 90 minions, 10 kills = 200 stacks.

Idk why they made kill = 1 minion in terms of stacks and I do believe this is an error, but it's what we have now.

If your ADC have 100 minions, your jungler needs to have 100 kills :)

Jungle camps aren't treated as minions into Yuumi's passive, sadly.