How many controllers are there per frequency? Imagine multiple controllers talking on the same frequency would be a nightmare for pilots and other controllers. 

So in real life how many controllers are there for each frequency? If there are more than one, how do they split the work load?
There is one frequency per sector and there can be two controllers per sector, one working the scope / talking to aircraft and one assisting with flight strips and coordination with adjacent sectors. If the sector does not require a second controller to coordinate and assist with flight strips, there will only be one controller doing all tasks single-handedly.

In some cases, two sectors are combined and both frequencies are manned by the same controller, but this is only done in low-traffic times, where overlapping transmissions have a low probability.

A good and well-written source to understand how ATC sectors and split workload works is the AVWeb Series of Say Again? by Don Brown.

See this related question for more information:
What is the maximum number of planes a controller can control?