Is Nature altar a elemental altar (Explorer ring use)?

Many nats do people average when rcing them with 91 and Graahk?

No - elemental is air, fire, earth, and water.

As for how many, it should be double at 91.

As far as how many nats per trip, it depends on how many pouches you intend on using. You can use the three pouch method (medium, large and giant) and you only have to do 2 crafts and tele out. I am not sure with the current improvements to pouches and the ability to fill them at the bank will make using 4 pouches more effective.

It is worth noting that Astral runecrafting is currently more profitable, and from my experience slightly better xp/hr.

 

Your questions have already been answered, so I won't repeat what others have said.