C is relative to the amount of current the battery is rated to handle, generally most brands will post several numbers related to "C rating"

100/60C will tell you that the pack can handle 100C "Burst" for a short period of time (typically a few seconds) and 60C for continuous.... some brands define continuous as a 30 sec burst, which really isn't continuous... facepalm!

So if you are running a 5000mAh pack, that converts to 5Ah, you simply multiply the C rating to the pack capacity to see how much current it can support, i.e. 5Ah x 60C = 300A of continuous current, but lets get real, there is no such thing as any 1/10 car that can continuously pull 300A of current, that would fully drain the battery in 60C / 60 sec = 1 min, that would be insane to expect a pack to be able to drain in just 1 minute without catching on fire!

If the brand doesn't specify what charge rate then you assume 1C, so for a 5Ah pack you set the charger to 5A charge rate.
Many brands of batteries will support up to 5C rates, so 5Ah x 5C = 25A though many racers will push this limit and charge their packs at 40A, more info here, skip to 6 min into the video: