[DonRotolo]

Or put another way: Use as many as you think you need so it doesn't fail.

Rivets are pretty strong, probably more so than the material being riveted. If I were building a bumper frame around my robot, I'd use rivets only in the corners where I need to join metal. If I wanted to hold 2 long pieces of metal together, I might use a rivet every foot or two and see what happens.

But the real answer is what ,4lex S. wrote: Analyze (aka "Do The Math")