How many palms form a round of applause?

Technically, two. That's because, according to the dictionary, round of applause is the same as applause – the plural of applause. So from two applause, we already have a salvo. However, a round of applause can also mean ovation, public acclaim. That is, in this case, if the palms are excited, they see a salvo.

I think 10-20, depending of the situation

Asking a question and giving the answer by your own? strange

5,it's a mathematical theory