FL Studio can use as many cores as your CPU has. There are caveats, the number of cores used depends on the number of parallel audio processing 'tasks' FL Studio can do in your project. You have some influence over this.
Create multi-core compatible projects - Make sure that your highest CPU using plugins are routed to completely independent Mixer Tracks without any shared 'Send' Channels. Multi-core CPUs need computational tasks that can be run simultaneously and so split across cores. Each Mixer Track represents an 'opportunity' to create these independent tasks. Each unit in the audio chain from the instrument through to the Mixer track and the effects must be processed in sequence over time. If one mixer track is linked to another, then all the instruments and effects on both Mixer Tracks now have a dependency and can't be split across cores efficiently.
You can read more on CPU and Memory in the FL Studio manual here. This section of the manual has some very important information that explains the difference between the operating system CPU meter and the FL Studio CPU meter.
You can help prevent speed-stepping and CPU core-parking by switching your CPU to 'High performance mode'...
Finally if you want to get deeper into this subject, here is a technical video on the matter:
Image-Line Support Team