I just want to go for the day and want to use the mineral hot spring tubs only. Where do I go? And how much does it cost for 2 or 5 people?

Go to the resort office. It may be best to call ahead and reserve a hot tub, as they go fast. They charge per tub, per hour, not per person. It's between $45 - $60 hourly, depending on time of day and location of tub.

Go to the resort. They charge you hourly

https://www.nchotsprings.com/ All the info you need will be on that site.