Is there a plugin for limiting classes?
For TF2, most of the plugins you're going to run into rely on a third-party scripting host.
One such popular scripting host is SourceMod (and its base library, MetaMod: Source).
SourceMod is fairly simple to install, but be warned: If you want some of its more advanced features, you need to add admins.
Once you have SourceMod installed, you can get the Class Restrictions mod from AlliedModders Forum. This allows you to set how many of each class there is on each team.