Each day, Alice is willing to pay up to $20 for a first cup of coffee, up to $18 for a second, and up to $15 for a third.

Each day, it costs Bob $11 to produce a first cup of coffee, $13 to produce a second, and $15 to produce a third.

Each day, Alice buys three cups of coffee from Bob for $15 each.

One day, the government imposes a sales tax of $9 per cup of coffee.

With the new tax in place, how many cups does Alice buy from Bob each day?