I've had some high altitude props on my shelf for a long time. I finally opened them today to find out there are only two props in there.
Do I need to buy another set to make use of the high altitude props? Or am I supposed to mix them with the regular props?

Why would they sell them in pairs if your supposed to have all four?

Frustrating as I am heading up to the mountains tomorrow and I am not sure if I will be able to fly my I2 or not.

you need 4, and all props DJI sell come 2 in a box. its so you can have 2 extras incase one breaks

Normal property will not stop you flying.

I flew at high altitudes in bighorn mountains and i was fine with normal props

I have flown the standard props at 6500 feet without any problems at all. Normally fly at 1350 feet routinely.

Don't think you will need these props this time. Yes, you need to order another pair to use them

All the best
Rich

As the guys have stated above you will need four and DJI sells all of their props in pairs. I think they do that with the thought that you may have damaged one and need to replace one or two and not go to the expense of having to buy 4.

I live in higher altitude and use normal props for now and just recently ordered high altitude ones and they should be here this week.  I just want to use them because I live in a higher altitude.lol  and DJI only sells them in pair.

Barrett, sorry for the late response, Inspire 2 Quick Release Propellers (High-Altitude) cannot be mixed with the standard propellers.
If you fly the drone at high-altitude (2500-5000m) locations, please use high altitude props for better performance, thank you.