I'm about to pull the trigger on a 6.5 Grendel upper.  I have a 6-24 Vortex that I'm going to throw on top.   I was wondering though, will I need a 20 MOA base to be able to reach out to 1000 meters?

Look at how much adjustment is in the scope.  A 60 MOA scope would give you 30 MOA of adjustment up.
Since you will probably need around 40 MOA for 1000, in this example yes you would need a 15 or 20 MOA base.

I have a 6.5-20X50 Vortex Viper and I normally shoot at 350' ASL.  I needed a canted base to get to 1000 yards with my 20" 6.5 Grendel.  I bought an Aadmount 20 MOA mount and I'm completely satisfied.  On my 16" 6.5 Grendel I'm using a Leupold Mark AR 3-9X40 in a RRA Cantilever mount with no cant.  I can get enough elevation to get to 700-800 yards at 350' ASL depending on the load.

You can run the ballistics for your elevation, 123 grain A-Max or SMK BC is 510, 20" barrel will give 2500 FPS, you then devide the max available elevation on the scope, ie 70 MOA adjustment you'd have enough scope for roughly 35 MOA bullet drop.   You didn't mention your barrel length but my 16" 6.5 Grendel gets about 2435 FPS with factory Hornady A-Max.

I got the Nikon XR 20moa and was concerned I couldn't get a 100 yd zero. Worked no problem. Had lots of room left at the bottom. Have not had a chance to get past 200 yet with a Bushnell 10x.

How much barrel is this Grendel upper you're getting going to have?

And which 6-24 Vortex are you using?  The Vipers have 65moa of elevation.

Hopefully LRRPF52 will chime in here.