Hi,
Not sure why this is tagged as PHP. It makes no sense to me to develop this kind of application using a web development language (unless you're expecting to have a webpage to do so, which makes the clarification that it should work on mac irrelevant).
The "Twitter way" of doing this is by means of the API. You login to Twitter and set your account to work with the API. Then you get your API credentials (tokens) from the Twitter page and then use them to operate on your account.
The application will ask for those credentials (which are needed to use the API) and also the twitter username to extract the followers from. The software will start following them one by one and will wait for a predefined amount of seconds before following the next one. It is not possible to know the amount as that is something that only Twitter knows and can change at their will but I'd say following 1 user per 15 minutes should be enough.
I can deliver this as a Java program that will work in Windows, Linux and Mac. The only requirement is that the computer has Java installed. It will also be a console application that will output progress to the console. If a user interface is needed a different price will have to be arranged.
Regards.