With more services being run on a server that is virtualized you will need a lot of switch ports. Our servers have ten one Gig ports plus 10 one Gig ports for the SAN. If you run firewalls and physical appliances you will need switch ports for these devises. If you are running virtual host servers you will want redundancy so you will need two switches. If the switches are not stackable you will need to trunk them and this will burn up four port per switch.
When we open up a city and deploy a cluster we use 44 ports right away. 30 for servers, 10 for the Storage Area Network, 2 for our remote PDU's and 2 data center cross connects. If the data center facility is doing maintenance to their infrastructure you will not go down. That is 22 ports per switch which leave us 26 ports per switch for expansion. This will leave room for about four or five more servers to add to the cluster. Buy 48 port switch if your budget allows, your cost per port will be lower. When you order your switches don't forget to order a stacking cables if not already included.