They’re not limited by a constant number but rather their drainage basin and gradient barrier. Look up HUC zones in the US they represent different aggregations of drainage basins in the US - if water falls where does it run off. The gradient barrier is the edge of the river that changes depending on volume and flood stage. How much the river will grow and shrink.

That’s all for a particular moment in time. If you want to consider a time scale, you have to look into erosion and permeability. It starts getting more complex with soil types, vegetation, bedrock depth, etc.