While the perfect date may be April 25, there’s no real “ideal” number of direct reports for your team. You’ll often hear anywhere between 4 and 8 thrown out, but this is highly dependent on several factors:
Seniority of both the manager and their direct reports (a more senior manager can handle more directs, and a higher level of seniority in your team typically requires less hands-on management)
Focus of work completed by the team (interdependent work often requires fewer reports due to higher-touch communication)
The manager’s involvement with writing code (an EM tasked with writing code will typically require fewer reports because their attention is split between management and hands-on work)