But we know where all the planes are, and actively monitor and move them around to avoid collisions. If there are a sufficient large number of satellites would there not be some risk of accidental collision with spacecraft taking off?
Every satellite is in a very deterministic orbit which requires energy to change (enormous amounts of energy for a significant change) so they don't change their orbit significantly nor often.
Ref: https://sos.noaa.gov/catalog/datasets/space-trash-and-satell...
We really don't. We mostly know for commercial flights, but not so much for general aviation. What aviation does is have zones around airports with restricted airspace that is well-controlled.
This is also true for satellites.
Do you think they don't know where their satellites are?