Does using my iphone's hotspot take up anything on my data plan?

My dad believes that it charges it in mass quantities, while my mom thinks it only charges a bit because it switches to 3G, with no extreme data streaming from the hotspot.

No because if you have a data plan you should be signed up for a monthly Internet? If not sure call your courier and ask

Hess it uses a fair bit of data depending on how much you use the other devices hotspot

If I'm correct it really all depends on what the devices that are tethered to it are using the data for.