WhatsApp doesn’t need staff because they weren’t processing regulated financial transactions. Thr app operated in a best efforts basis since it was mostly free. You don’t need customer support staff for that — there is no support.
FWIW, around 1/3 of the 55 were customer support. That's not a lot of support per user, but it's not none. And it is enough to get lots of feedback to engineering about things users are having trouble with, because the better you make the product, the less overloaded customer support is.
