Peak number of concurrent sessions

Table sessions:

Find the peak number of sessions running at the same time. A session that ends exactly when another starts is not running at the same time as it. For an empty table the peak is 0.

Output: one row, column peak.

CREATE TABLE sessions (
  id int PRIMARY KEY,
  start_at timestamp NOT NULL,
  end_at timestamp NOT NULL CHECK (end_at > start_at)
);