Somewhere between 150 and 160, depending on the tennis balls. Hope this helps

https://www.wolframalpha.com/input?i=9kg+%2F+mass+of+a+tennis+ball

Edit: Additionally, that's about 63½ European swallows, assuming an average weight of 5 ounces. Given that a European swallow must beat its wings 43 times per second to maintain airspeed velocity, it'd be a proper racket.

It's seventy-nine sticks of butter, plus a pat or two

The only units I understand are bananas or bald eagles. Please adjust accordingly

I used robots and the answer was 160 tennis balls, which is actually much less than I expected.