One sitting, three possible outcomes
You do not register for level 7, 8 or 9 separately. One exam is sat and the result places you into one of the bands the standard defines, or below the entry band. Scoring is by item response theory, with each of the five skills scaled onto its own report score, so a strong translation result does not disguise a weak reading one — every skill is visible.
Because the scaling is IRT-based, raw item counts per skill are not published, and this page states none. What is published — the whole-exam question count, the approximate duration, the delivery mode and the band names — is printed with its source.