* Tigers are generally solitary animals. They don't typically hunt in packs like lions do.
* "Ambush" implies a coordinated effort. While a tiger might ambush prey on its own, it's not common for multiple tigers to work together in this way.
So, the answer is likely one. However, there have been rare documented cases of tiger mothers hunting with cubs, or of unrelated tigers cooperating briefly for a shared meal.
It's important to remember that these are exceptions, not the rule.