You just need one router going to your ISP, even so ISP provided router is suitable enough. You will need at least 1pc 24 port switch and 2pcs 16 port switch for your LAN compose of 30-50 PCs. Unless you have a public IP lease from your ISP and hosting your own public game servers you will not need a high end routers/switches. if the intent is just LAN games or MMO games then make it simple and just pour your resource in getting the best rate for intermet connection.