Posting just the link to PubMed is not very helpful. I agree that many questions on RG are very general and with a bit of self initiative the inquirers would find the answers on their own, but this is a specific question and in fact not easy to answer.
In my opinion the best density would be 5,000 to 10,000 cells/ 10 uL (drop) inside the 96 well hanging drop plate. This will give you enough number of cells so you can see the expansion of spheroid over 7 days period.
I used this number with the protocol available at Sigma Aldrich website http://www.sigmaaldrich.com/technical-documents/protocols/biology/perfecta3d-hdp1096-media-exchange-protocol.html