Long Island man wearing 9kg-metal necklace dies after being sucked into MRI machine
(www.independent.co.uk)
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.
Tap for spoiler
Those numbers are from monty python and the holy grail and are very wrong. I am spreading misinformation online.