Job interview 'thank you note' etiquette?

Recently I’ve been interviewing for positions that have upwards of 6 rounds of interviews, some of them with panels of three or more people per interview. I’m used to sending ‘thank you’ notes after interviews, but do I have to send a thank you note to every single person in each interview, and for each instance? We’re talking about 20 potential thank you notes, here.

Also, what’s the best way to include the HR Manager in the thank you note process? Do they get one? Each time as well?