Contested Game Ball's triggered ability triggers only once whenever you're dealt combat damage, no matter how many creatures deal combat damage to you at the same time.

In a Two-Headed Giant game, each player on the attacking team is an attacking player. When Contested Game Ball's first ability resolves, you'll choose which member of the attacking team gains control of it.

If point counters are put on Contested Game Ball some way other than its last ability, you won't sacrifice it or create a Treasure token. The check for five or more point counters happens only as the activated ability resolves.