There is no single equation for calculating the magnetic field produced by multiple charges in free space because the problem is inherently complex. Instead, the magnetic fields from individual point charges can be summed, taking into account a retardation factor due to the finite speed of light. This means that changes in the magnetic field are delayed, affecting the overall calculation. In small regions, this time delay can often be considered negligible. Understanding these principles is crucial for accurately modeling magnetic fields in various scenarios.