Short answer
There’s no single right number. A good span of control depends on how similar the work is, how experienced the team is and how much else the manager is responsible for. A manager leading experienced people doing similar work can handle more direct reports than one leading new hires on varied, cross-functional work. The better question is whether your managers have the skills and time to coach the team they have.
The longer answer
Spans often widen after a restructure, without anyone adjusting what the manager role requires. The manager who used to coach eight people now has fifteen and the same calendar. Coaching is usually the first thing to go.
Three things help more than redrawing the org chart. Train managers to delegate decisions so fewer come back to them. Build team leads who take on part of the coaching. And cut the meetings and reports that don’t need the manager. That’s how a manager works within a wide span, and how they grow into a wider one.