Is publishing papers with many co-authors detrimental to one's academic career?
As mentioned in comments, the normal number of authors on a paper depends heavily on the field.

The issue with getting a postdoc is that you need to demonstrate your ability to do research. That doesn't necessarily mean you need to be the PI, as in some fields as a postdoc you would still be working on projects driven by someone else. 

Having one or more individual papers would be one method of doing that. Being lead author on a paper in a field where order denotes role would be another. But for a PhD student applying for postdoc positions, their references will still play a big part in whether they are hired. Their supervisor should be able to include in the reference what part the student contributed to the paper.