Which flowers have seven petals?

No there is not.

The number of petals is directly related to the type of plant:

Monocotyledonous plants have floral parts (eg. petals and anther's) in multiples of 3 (3,6,9,12 etc.)

Dicotyledonous plants have floral parts in either multiples of 4 or 5 (4,8, 12 etc.) and (5,10, 15 etc.)

So typically there cannot be a flower with 2, 7, 11, 13 petals.