I feel that 12 sets of 7 reps is generally near optimum considering WORKLOAD (weight * TUT) and taxes both the sarcoplasmic and sarcomere components.
Occaisonally, when hitting plateu you should build more strength (after all, progressive overload is the key for long term gains) and do for example:
5 * 5 or 5 * 3
followed up by
7 * 9 (just to maintain your sarcoplasmic system)
which totals 12 sets again.