In the game of sequence: A sequence of more than 5 coins
You don't have to declare which row of five is the sequence, as any row of five is protected by the rule:

You cannot remove a marker chip that
is already part of a completed SEQUENCE. Once a SEQUENCE is achieved by a player or a team, it cannot
be broken.

Where a Sequence is defined as:

A Sequence is a connected series of five of the same color marker chip in a straight line, either up and
down, across or diagonally on the playing surface.

Together, these rules prevent any connected series of five chips from being broken.
As mentioned in my comment, any series less than 9 is only valid as one sequence for scoring. A series of 9 can count as two sequences overlapping by one coin.
Rule Source