I'm building an Amazon EC-2 cloud-based build server for my personal development projects using Team City Professional. I only intend to use it of an evening and my project builds are going to be quite small.
At this stage, is there a good reason why I should go for two servers:
How many servers you need or which instance type to use heavily depends on the nature of your projects and the number of resources they utilize during the build -- add to that how fast you want your builds to be done and how much money you want to spend.
Generally, it sounds like you want a EBS-based instance so you can stop and resume at any time. I don't see how two servers right from the start would do you any good and start with one and scale up by going to a larger instance size before adding a second server to the mix.
If you keep the server online only for a few hours a day, then the cost should be reasonable anyway.