How many controllers are there per frequency?
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?