For your planned environment (if you do not want to build a cluster of servers) 2 Pro licenses are sufficient. One is for the productive server, another one for your development team (and test servers).
On the other hand I see no big reason to create the pdf generation server. You may simply drop pd4ml.jar and ss_css2.jar into lib directory of your application and have the PDF generation functionality in-place. In the case you do need the Volume license, and it is worth its money if you compare it with efforts to establish and maintain the PDF-generating network infrastructure.