Also depends on the company and how they license their products. There is places that lets you install a copy of a program on more than one machines but you are the sole person thats allowed to use it on all the pc's. In most cases however you will need one license for every machine you have it installed on unless you have a volume license key.
Its hard for them to monitor but these days programs have the ability to go online (where there is a connection) and authenticate itself against a database and if it can't the program will be disabled.