You do need somebody to develop the open sourced system in the first place. Of course having a system which is open source can have efficiency advantages as in

You do need somebody to develop the open sourced system in the first place. Of course having a system which is open source can have efficiency advantages as in theory as there is less need to re-invent the wheel. This doesn't necessarily seem to happen in practice though, how many competing open source web frameworks are there? I don't think you can really say there is a correct number of developers that you "should" have. For example most of the software projects I have worked on "should" have had hundreds of developers, because we would have had to build not just the app but the surrounding infrastructure such as HTTP server , DBMS etc. When productivity is increased by easy access to open source, the scope of projects tends to go up too. Back in 1999 a website that had some HTML text and a few images was generally good enough. Now people want sites with social integration , full text search , streaming video etc so even though the parts are easier to fit together there are more of them.